Abstract: The electron beam of high average power Free Electron Laser Terahertz scientific research device in Chinese Academy of Engineering Physics (CAEP) has the characteristic of high average current. Non-destructive measurement technique is required for the measurement of the beam parameters. The beam position monitor (BPM) is one of the basic measurement instruments. The method of extracting the signal spectrum of the BPM to obtain the bunch length is simple and can be used as a supplement or replacement of other bunch length measurement methods. This paper analyzes the frequency domain characteristics of the driving electron beam bunches, and analyzes the theoretical basement of the bunch length measurement based on the BPM. 3D simulation program was used for the numerical simulation of BPM transfer impedance modeling, the bunches of different length were measured and calculated, and the impact of the bunchs off-center position in the vacuum pipe on the bunch length measurements was also analyzed. The study results show that when the bunch length varies from 10 ps to 100 ps, all the measurement error is less than 2%, which meets the requirements of CAEP-THz-FEL operation.