Abstract: A cluster-restricted pilot-free spread spectrum despreading algorithm is proposed for long-range underwater acoustic communication in deep sea. Utilizing the characteristic that the deep-sea channel structure changes slowly within a period of time, the channel prior cluster area information obtained by the pre-transmission pilot sequence is used as the constrain in the first step of pilot-free channel estimation (PFCE) algorithm to divide the channel estimate result into channel and non-channel regions. The experimental results show that the proposed method outperforms the traditional channel equalization despreading algorithm, and has a significant improvement compared to the PFCE algorithm. Compared with the PFCE, the proposed method can reach the better performance without subsequent iterative channel equalization, lower the computational complexity, and reduce the average bit error rate by 1 to 2 orders.