In view of the disadvantage that coal gangue powder can reduce the crack resistance of asphalt mortar at low temperature, silane coupling agent KH 550 was used to modify coal gangue powder. Coal gangue powder asphalt mortar and limestone mine powder asphalt mortar modified with five dosages of silane coupling agent（0%,05%,10%,15%,20%）were prepared under different filler asphalt ratios. The performances of the asphalt mortars were compared and the mechanism of modification of coal gangue powder with silane coupling agent was analyzed. The results show that after modification with silane coupling agent, the high temperature performance, low temperature performance and adhesion of coal gangue powder asphalt mortar are significantly improved, and with the increase of the amount of coupling agent added, the surface of coal gangue powder becomes more rough, and more interstitial pores are formed, which results in the larger contact area between coal gangue powder and asphalt. Moreover, a polysiloxane coupling layer is formed on the surface of coal gangue powder, which improves its adhesion with asphalt, thus the road performances of the asphalt mortars are significantly improved.