Aiming at slow speed of the long PN code acquisition in DSSS, the similarity between the short PN code in series and the long one can be used to improve parallel digital matched filter(DMF) for the acquisition of PN codes. An improved and simply practiced acquisition method which combine with serial and parallel processing is got. And then a simple and practical adaptive threshold is choosed by using the correlative value of PN code in a period and iterative mode. By using this adaptive method, it is not necessary to estimate the noise and the calculation load is small. Simulation demonstrates that PN codes of the proposed method can be detected at higher acquisition probability and has a better ant-inoise performance.