In this paper,the tolerance of the end mirror(plane mirror) tilting in high speed rotary spectrometer is analyzed systematically by means of modulation depth and phase error. It is found that the changes resulted from the tilting of the end mirror was made up of three parts: 1) the constant OPD that reflective light increases: 2) the increased OPD concerning different ray along the perpendicular direction to light axis: 3)the moving distance of the light beam center to light axis. The SNR of the output is reduced by these changes. The formula of changed interferogram intensity is given. Methods to avoid the problems resulted from the tilting of the end mirror are also discussed.