Many authors, Yamamoto et al., Byskov, Rao et al., Walsh, Tong et al., etc. have discussed the availability of finite element methods to the calculation of stress intensity factors of cracked plates. Some of them successfully tried the use of special finite elements at the crack-tip-region in order to obtain a higher accuracy. But these are not always very easy to apply to the practical problems because of the complexity of calculation models. The present paper surveys some general relations among variables in finite elements, and proposes a substructure model, as a general extention of Walsh's method, which has a large liberty of choosing crack-tip-element characteristics and reduces the dimension of stiffness matrix. Some simple numerical examples are given to verify the utilities of this model and to examine the influence factors to the accuracy of solutions.