Two kinds of alignment errors between the gyro coordinate system and platform coordinate system were introduced when Electrostatic-Suspended Gyroscope(ESG) was used in measuring drift angle on servo turntable by Position Sensitive Detector(PSD). Then formulas for calculating the coordinate change of spot center of reflected beam on PSD caused by the alignment errors were derived

and the calculating methods of servo following errors were presented when alignment errors existed between the gyro coordinate system and platform coordinate system. According to the structure and the working condition of ESG

a group of data were chosen to calculate the servo following errors. The computational results show that the first error is two orders of magnitude larger than the second one. In order to reduce the system servo following error

the first error must be less than 4'. This conclusion plays an important theoretical guiding role in ESG designing and testing.