GPS L1C navigation signal is a representative of new system signals, whose navigation message in the IS-GPS-800 protocol is not fixed frame header structure. Because of the data has no constant frame header, traditional algorithms of frame synchronization unable to be used. Based on the characteristics of L1C signal, a common algorithm is correlation function algorithm of the overlay code. The following aspects are studied in this paper: Performance of this algorithm under low SNR; How to determine the detection threshold. In this paper the complete overlay code is used to analysis the relationship between SNR and success rate of frame synchronization.