To study the mechanisms of strength of cushion material after freeze-thaw cycles in cold environment, a series of cushion material samples with different gradation after one freeze-thaw cycle have been tested on direct shear apparatus, test results show that: the cushion material strength indicators number vary with the pore size and the pore quantity. The relationship of friction angle and equivalent pore quantity is linear, and the slope of the regression linear increases with increasing of equivalent pore size; the relationship of cohesion and equivalent pore quantity is also linear and the slope of the regression linear decreases with increasing of equivalent pore size. It has been found that the strength of cushion material after thawing is not altered and the permeability can be adjusted by changing particle content under a gradation range.