The invention discloses a method for reducing periodic error in a rotating machinery torsional vibration signal pulse measuring method. The method for reducing periodic error in the rotating machinery torsional vibration signal pulse measuring method comprises the following steps of A, installing a marking device and a detection device at a rotary shaft measuring section and enabling the detection device to stably output a correct waveform; B, performing signal sampling through a high frequency counter and recording time interval between any two continuous pulse signals output from the detection device; c, calculating instant torsional vibration angular speed at any point according to an instant torsional vibration angular speed calculation formula; D, calculating a periodic time delay average function of the marking device during the process that the marking device rotates for a round according to pulse signals; E, expanding a periodic time delay average function of any point according to the periodic time delay average function; F, deducting the periodic time delay average function from actual measurement vibration signals output from the detection device and obtaining torsional vibration signals after error compensation. The method for reducing periodic error in the rotating machinery torsional vibration signal pulse measuring method has the advantages of overcoming defects in the existing rotating machinery torsional vibration signal pulse measuring method, effectively reducing error of division and bending vibration influences to a torsional vibration measuring result and improving the accuracy of the torsional vibration measuring result.