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Aiming at reducing the system complexity, raising communication rate and supporting CDMA based applications, a direct sequence spread spectrum based underwater acoustic communication system is proposed. BPSK/QPSK modulation is used. A RA coding scheme is adopted to reduce the BER. To reduce the phase-acquisition complexity of receiver, a carrier modulation method which lets spread-spectrum code and carrier have the same phase is adopted in transmitter. To raise the communication rate, an architecture of de-spreading before demodulation is used in receiver. To support CDMA based applications, a band-pass signal acquisition strategy is used in receiver, and the acquisition method is delay correlation acquisition algorithm. Matlab-based simulations and practical system based experiments are carried out. The results show that the system has a good performance at supporting multi-user communication and rejecting multi-path effects. Under a water tank environment, the BER of the system without interference of other users is at 10-4 order of magnitude, and the BER of the system with interference of two users is at 10-3 order of magnitude.