Abstract: Based on the influence cases of extensometer jitter, poor clamping, droop and inaccurate positioning on the measurement of plastic strain ratio (r value), the reasons for the deviation of r value measurement results were analyzed by combining with the calculation formula. The results show that the abnormal measurement results of r value in the case were caused by the asynchronous measurement feedback of longitudinal and transverse extensometers. Methods and steps for finding measurement errors were provided,and reasonable suggestions were put forward.