To improve the accuracy of assembly contact analysis, the contact state between mating surfaces with form error was studied. With the plane as object, the contact state analysis method of mating surfaces considering the form errors and assembly forces was proposed. On this basis, the concept of mating variation and its calculating method was proposed, in to reveal the effects of form errors on the assembly accuracy. Subsequently, the finite element method was adopted to verify the proposed method. The results show that the method is effective, accurately predicts the contact state of mating surfaces with form errors, and calculate the complex error. The proposed method can provide a basis for prediction and analysis of the assembly accuracy when considering the form errors.