In the process of underground mining and tunnel construction，the rock is mostly in the process of repeated loading and unloading，and the macro-cracks and micro-defects in the rock mass itself will continue to expand under fatigue loading and eventually lead to many accidents.Therefore，it is of great engineering significance to study the micro-defects of fractured rock masses under fatigue loading.In the previous studies, acoustic emission and CT scanning technology were mainly used to reflect the change of microscopic damage through AE number or CT number, which obtained good results.The porosity measured by nuclear magnetic resonance technology can also be used to show the damage of rock samples，but it is currently mainly used in uniaxial compression loading.In this experiment，yellow sandstone specimens with five fracture inclination angles （0°，15°，30°，45°，60°） were selected.Firstly，uniaxial compression experiments were performed to measure the uniaxial compressive strength of fractured sandstone at various angles.Then carry out fatigue loading experiments，and select the upper limit fatigue loading stresses based on the measured uniaxial compressive strength to be 19.1 MPa,25.6 MPa and 30.3 MPa，respectively.The internal porosity of each fractured sandstone before and after 30 fatigue cycles under each upper limit stress was measured with nuclear magnetic resonance instruments.Combined with the analysis of T₂ spectrum measured by nuclear magnetic resonance instrument，it is found that with the increase of the upper limit stress，the total area of T₂ spectrum and the accumulation of porosity in fractured sandstone have increased significantly.This shows that its internal damage increases with the increase of the upper limit stress，and then combined with the T₂ spectral area for quantitative analysis，it was found that the spectral area of the small pores in the T₂ spectrum changed only slightly with the increase of the upper limit stress，while the spectral area of large pores increases exponentially with the increase of the upper limit stress.No matter from the angle of the change of the ratio of large pores (to the total pores) to the fatigue upper limit stresses under different inclination angles，or from the angle of the change of the ratio of large pores under different fatigue upper limit stresses to the angle of fracture inclination，similar rules are obtained.Therefore，it can be concluded that the change of large pores is the main factor that causes the total porosity of fractured sandstones to increase after fatigue loading，and then the curve of the ratio of the proportion of large holes to the upper limit fatigue stress ratio after fatigue loading at various inclination cracks is fitted.The fitting coefficients of sandstones at various inclination angles are very high，which shows that the change law of internal damage can be well displayed.Considering the effect of initial damage of rock samples on fatigue loading，two types of damage variables based on the ratio of large pores to total pores were defined to study the impact of fatigue upper limit stress on the fatigue damage of sandstones with different fracture inclination angles.According to the relationship curves between the two damage variables，the upper limit stress ratio and the upper limit stress，the change of the defined damage variable can be well projected. Further analysis can show the intensity change of the sandstones with different fracture inclination angles，that is，the damage intensity of the fractured rock sample that tends to be gentle will eventually be greater than that of the fractured rock sample with a sharp increase in damage. Finally，combining the probability density function of Weibull distribution and generalized Hook’s law，it is deduced from the theoretical formula that it is reasonable to use pores to study sandstone fatigue damage.Combining the results of previous studies and this experiment，it was found that the porosity can well reflect the microscopic damage of rock samples in both uniaxial compression experiments and fatigue loading experiments.