In order to better highlight the application advantages of passive radar system such as safety, reliability, strong anti jamming and strong concealment, this paper firstly constructs a passive radar system based on civil mobile communication signals, and analyzes the sources of passive positioning errors and positioning accuracy evaluation standards. Secondly, the implementation process of two classic TDOA positioning algorithms, Chans algorithm and Taylor series algorithm, which are suitable for this system, is analyzed. Finally, under different base station configurations, the Calamero lower bound of positioning accuracy of the two classical TDOA positioning algorithms is simulated and compared.The simulation results show that when the mean square error of the measurement error is low, in order to better approach the lower bound of Calamero, technicians can apply any of the above TDOA positioning algorithms to deal with the measurement error, so as to effectively improve the performance of the TDOA positioning algorithm.