Abstract: By calculating the cross ambiguity function of the reference signal and the echo signal based on the passive coherent location technologies, passive radar detects the distance and speed of the target. However, it is difficult to meet real-time requirements when calculated the cross ambiguity function directly. To solve this problem, this paper proposes a method that uses the rectangular window as the first stage filter of a multi-stage decimation based on two types of fast calculation method: delay traverse and Doppler frequency traverse, which are based on Chinese National Standard Digital Television Terrestrial Broadcasting (DTTB) signal. Compared with the direct calculation and the non-decimation calculation method, the amount of the calculation can be reduced more than 3 orders and 1 order separately. This paper further analyses the variance of the calculation amount of the two types of fast calculation method when time accumulation is changing and provides a reference for the choice of calculation methods in practical applications.