Abstract: In order to improve the accuracy and stability in the measurement of slightness hole system in zero-lock laser gyro cavity，the main error in the measurement of slightness hole system in out-of-plane zero-lock laser gyro was analyzed. The expression of the effect of inclination error on the projective angle of spatial out-of-plane hole system was built. It was found that the variation of inclination of spatial out-of-plane hole system could change the projective angle , when the inclination was 16 ，30% of the inclination error weighted in the projective angle error，and it was verified by UG software. Moreover, the effect of measuring point error from video measurement instrument on projective angle was analyzed with the method of least square fitting, and it was found that increasing the measuring points from 2 to 10 could reduce the influence of super error by 50%. Results show that this method increases the stability of the zero-lock laser gyro measurement.