The improvement of the heterodyne laser interferometer accuracy is strongly restricted by the periodic nonlinearities which arise from the optical mixing in the measurement and reference arms. Imperfect laser polarization is a principal factor leads to the optical mixing, which can cause the first- or second-order nonlinearities, or both of them, but the transformation mechanism of the two nonlinearities is still ambiguous. Starting from the nonlinearity model based on optical mixing, this paper derives the nonlinearity expression with the two-frequency laser polarization parameters, which is applied to analyze the transformation mechanism of the nonlinearity harmonics. Simulation results shows that the coincident degree with the orthogonality of Jones vectors of the two laser components determines the existence condition of the first- and second-order nonlinearities, i.e. when the orthogonality is satisfied, the error caused by laser polarization is the second-order nonlinearity; when the orthogonality is far dissatisfied, the error caused by laser polarization is almost the first-order nonlinearity, whose magnitude is generally one order larger than that of the second-order nonlinearity; beside the above-mentioned two conditions, the error caused by laser polarization is composed of the first- and second-order nonlinearities.