Abstract: In order to accurately understand the effect of pump on/off on bottomhole pressure and provide guidance for safe drilling,a model of bottomhole pressure fluctuation caused by pump on/off has been built according to unstable flow theory.The authors analyzed the process of transmission and transformation of pressure waves in the well by means of the solution of characteristic line, and computer programming and well simulation results using field experimental data.The effect of duration of pump on/off,the distance of drilling bit to the bottom fo the hole and the drilling fluid properties was also studied.The simulation results showed that the longer the duration of pump on/off,the smaller the abnormal pressure window ratio;the greater the drilling fluid density,the larger the abnormal pressure window ratio;the larger the drilling fluid consistency coefficient and flow index,the smaller the abnormal pressure window ratio.The study indicated that duration of pump on/off,drilling fluid consistency coefficient and flow index of drilling fluids had a conspicuous effect on bottomhole pressure.The model of pressure fluctuation caused by pump on/off during drilling can provide theoretical reference for starting and stopping the pump on site and can predict mud pump damage.