Vertical joints with different lengths and slope distances (the horizontal distance from joints to slope shoulder) are generated at the top of the loess slope because of the unloading and collapsibility of the loess. The existence of vertical joints is a significant risk factor for loess landslides. This paper applies three methods, including laboratory tests, numerical simulation and theoretical analysis, to analyze the influence of vertical joints on the stability of loess slope. Firstly, the typical failure mode and strength characteristics of the samples containing vertical joints are analyzed through the unconfined compressive strength test and triaxial compression test. Further, the effects of vertical joints with different lengths and the slope distance on the slope safety coefficient and sliding surface position are calculated by finite element numerical analysis. Finally, the formula for calculating the safety coefficient of loess slope with vertical joints is derived. The results show that: the joint bonding degree affects the sample's strength and failure rate. The strength of the sample with open vertical joints is less than that of closed ones, and the failure rate is greater than that of closed ones. When the length and the slope distances of the vertical joint are less than 5 m, the influence of vertical joints on the shape and position of the sliding surface will be small. When the length of the vertical joint is more than 5 m and less than 15 m, the safety factor of the slope decreases with the increase in the length of vertical joints. When the length of vertical joints reaches 15 m, and the slope distance is 10 m, the deterioration degree of vertical joints on the slope comes up to the peak. Statistical analysis shows that the dominant dip angle of shear fracture at the back edge of the landslide affected by the vertical joint surface is between 45° and 65°. The research results can be used for rapid calculation of the safety coefficient and rapid evaluation of slope stability of loess slope with vertical joints.