Abstract: The stress intensity factor for the crack at the bottom of asphalt layer with supposion of the completely continuous interface is got using the center crack subjected to normal tensile stress as the reference state according to weight function mechanism and Petroski and Achenbach’s method. The equivalent friction area on the interface of asphalt layer is got by the equivalence between the stress intensity factor supposing that the interface is completely continuous in a critical static slide state, so the slide friction force and the stress intensity factor can be got.The stress intensity factor considering partial constraint on the interface can be got by the superposition between the stress intensity factor for the bottom crack supposing the interface is completely smooth and that by the critical static friction force. The corresponding program is made to calculate the stress intensity factor and an example is also given.