It is well known that shear strength of soil plays a key role in analyzing the stability of slope. To clarify the variations of shear strength and parameters of the loess after leaching for different times, a series of direct shearing tests is conducted on samples from Lanzhou loess. Results show that, shear strength and cohesion of unsaturated loess decreases significantly nonlinearly with the decrease of soluble salts, and inner friction angle is slightly changed. Specifically, the reduction reaches the maximum value after leaching 1 time and trends zero after leaching 5 times. It is also found that the reduced rate is large when soluble salt content is greater than 2 620 mg/100 g and then reduces. The decrease of loess’s shear strength comes primarily from the decomposition of soluble salts and calcium carbonate at the initial stage, and it relies on the dissolution of soluble salts and weakening of matric suction afterwards.