Abstract: Considering that high computational complexity exists and it is hard to find the theory to refer when we select register state for the rapid PN code acquisition algorithm based on iterative massage passing (IMP), a modified rapid PN code acquisition algorithm based on IMP is proposed. Based on the analysis of the probability density function of posterior information, an evaluation function is constructed to measure the reliability of estimation vector of register state using a posteriori probability log-likelihood ratios (APP LLR). Then we select the estimation vector according to the reliability from high to low as register state to perform correlation test. If the correlation value is greater than the threshold value, the acquisition is successful and the acquisition procedure is stopped. Otherwise, the same operation is performed for the next estimation vector. If all the correlation values are smaller than the threshold, the acquisition fails. Finally, analysis and simulations are performed based on several m-sequences with large order. Results show that the complexity of modified algorithm is reduced to some extent, the convergence property is improved substantially and acquisition performance can be improved about 1~3 dB. This algorithm can be applied to the rapid PN acquisition, especially in spread-spectrum communication system with long PN code.