<p>This study presents a method for on-line measurement of spindle radial error motion during machine tool cutting. Measuring the spindle radial error motion during cutting is crucial for monitoring workpiece quality and analyzing spindle dynamic characteristics. The measurement during cutting is limited by the cutting process and process parameters. The problems encountered in measuring under cutting conditions, such as inaccurate data interception, angle non-uniformity of sampling data, and installation angle error, are simulated and analyzed. An on-line measurement method based on the built-in encoder signal of the spindle is proposed. The synchronous acquisition technology of angle signal of the spindle built-in encoder is used to reduce truncation error to improve the accuracy of data extraction and calculation without affecting cutting or adding sensors. The equal angle interval resampling method is constructed to convert the equal time interval data into the equal angle interval data to reduce the impact of signal angle non-uniformity on measurement. The simulation comparison shows that the proposed method can effectively solve the problems existing in the measurement under cutting conditions. Finally, an actual cutting experiment is carried out to verify the effectiveness of the proposed method, and it is shown that the method can not only achieve on-line measurement during the cutting process, but also weaken the influence of cutting conditions, and achieve higher measurement accuracy than the three-point method.</p>