Abstract: Vibration of several large vertical pump set motor in a power plant exceeded the standard due to structural resonance. During the vibration treatment by dynamic balance, it was found that the mechanical lag angle of the same type of pump unit with struct ural resonance had a large deviation. At the same time, it was also found that there was a large deviation in the vibration phase during the data acquisition of the same measuring point by two sets of portable data acquisition instruments. In order to dete rmine the reason for the deviation of the lag angle and the influence of the vibration phase deviation on the dynamic balance process, the reason for the deviation of the mechanical lag angle is determined through the comprehensive analysis of the spectrum data, natural frequency and critical speed. By theoretical derivation and actual data calculation, it is determined that the phase deviation will have an influence on the angle deviation of the test counterweight, but will not have an impact on the final correction quality.