Abstract: Nonlinear errors were introduced by non-ideal 1/4 wave plate and polarizing beam splitter (PBS) when using a homodyne Mach-Zehnder interferometer to monitor the optical frequency of a tunable laser. The error formula of the system was theoretically deduced by mathematically modeling the interferometer with Jones matrix. Theoretical analysis and numerical simulation indicated that the nonlinear errors of the system were the linear superposition of first and second harmonic wave errors, the first harmonic wave errors were mainly caused by the unbalance of the two PBSs- alignment errors, the second were mainly caused by wave-plate-s alignment errors, PBS-s alignment errors and wave plate-s phase delay errors, and the overall second harmonic wave errors were approximately the linear superposition of the errors caused independently by these three error sources, errors caused by polarization leakage of the PBS could be neglected if the leakage was small.