Traditional blind additive SS (spread spectrum) watermarking systems have such features: high robustness, low detection value and small capacity. The main reason leading to low detection value is the host signal’s interference. Based on the Gaussian signal detection principle under Gaussian noise, the correlation detection is analyzed in theory. In addition, the using condition of correlation detector and the relationship between the normalized correlation detector and SNR (signal noise ratio) are discussed in this paper. Based on these analyses, an improved blind additive watermarking algorithm is designed. Experimental results show that the algorithm effectively holds back the interference of host signal and the performance of detection has been greatly improved. Furthermore, it has greater robustness and imperceptibility.