Abstract: This paper introduces the approach to design a stripline beam position monitor(BPM) for the project of “THz High-flux Material Physical Property Testing System”, which is hosted by National Synchrotron Radiation Laboratory. Due to the non-orthogonal symmetric vacuum chamber at the exit of the undulator, two kinds of stripline BPMs with rectangular and racetrack vacuum chamber respectively are considered as an option, and compared with the traditional BPM with circular vacuum chamber. The modeling and simulation are based on the boundary element method via Matlab. The simulation results show that the sensitivities of BPMs with rectangular and racetrack vacuum chamber are improved by 30%, the impedance matching errors are reduced by 20%, and the beam position fitting errors are reduced by 80%, compared with the traditional BPM with circular vacuum chamber. Considering the accuracy of processing, the stripline BPM with rectangular vacuum chamber is more suitable for the project.