Abstract: To resolve the problem of micro gear or non-rigidity gear position alteration errors, a non-contact projection measurement method is adopted. Appling projection measurement appliance and rotary table by single pitch method and span pitch add-on spot method, through compensation of division errors of rotary table, gear pitch cumulative errors measurement is finished which is proved reasonable by absolute method measurement. Through analysis of the install gear eccentric errors, instrument measuring errors and aiming alignment errors, the gear pitch cumulative errors uncertainty is got about ±65μm by single pitch method and ±64μm by span pitch add-on spot method which all meet the demand of measurement precision. Finally, a conclusion is made that the tested non-rigidity plastic gear precision level is confirmed as 10 level by both measuring methods. By comparison, span pitch add-on spot method has more advantage in less measuring times and easy control in gear pitch cumulative errors measurement.