The effect on modulation depth caused by polish errors is analyzed from the interference principle of Michelson interferometer.The modulation depth cannot achieve ideally 100% due to surface errors, but the modulation depth cannot be below 90% in an ideal case. Through the analysis and discussion, it may be established that the surface errors should be plane to about λ/14 or better so that the modulation depth is not below 90% for satisfactory performance of the interferometer. At the same time, flatness for 5 mirrors is measured by Zygo interferometer so as to get peak to peak value and some other parameters.The flatness requirements of modulation depth is met according to the analysis of the measured results. Based on the study of the surface errors of the Michelson interferometer, it is a certain guiding significance to design, develop and analysis performance for an interference spectrometer.