By taking advantage of spread processing gain, the direct-sequence spread-spectrum (DSSS) for underwater acoustic (UWA) communication system can be carried out at low signal levels, which is the preferred method for high-quality UWA communication and remote UWA communication. However, phase fluctuation caused by complex marine environment seriously affects the performance of spread spectrum system, leading to the reduction of spread processing gain. Differential energy detector is proposed for DSSS UWA communication system in this paper, which has a good ability of anti-carrier phase fluctuation and multi-path interference by detecting the output energy of two correlators. Differential coding can avoid error propagation when determining the relationship between adjacent symbols. Differential energy detector combined with improved active average sound intensity detector is further proposed in this paper, which can get vector processing gain by updating the estimated azimuth so as to make the system operate stably at a low signal to noise ratio. Improved active average sound intensity detector also has the ability of anti-carrier phase fluctuation, and the feedback code bit information of differential energy detector can ensure that the processing gain of improved active average sound intensity detector is not affected by Doppler's accumulation. Simulation and Dalian sea test have verified the robustness of single vector differential energy detector algorithm. Using the single-vector differential energy detector, good performance is achieved for a signal-to-noise ratio as low as -18 dB based on at-sea data.