Abstract: It is not easy to obtain directly the stress field at the crack tip of I-II combined mode plane crack problem. In this paper, the Westergaard stress function is used to solve the plane crack problem of I-II combined mode, complete with the expressions of the stress components at the crack tip. The method is simple in the derivation procedure and explicit in the physical concept, and the results are shown to agree well with available results in open literature. At the same time, the superposition principle is applied to transform the force on the crack surface into the force on the outer boundary for the problem of I-II combined plane crack with force on the crack surface.