Accurate synchronization is more difficult in the time-variation channel. Conventional cyclic shift keying spread spectrum suffers performance impairment from timing errors in underwater acoustic communications. To solve this problem, a differential cyclic shift keying spread-spectrum underwater acoustic communication technique based on passive time-reversal was proposed. The proposed method requires only “coarse” synchronization at the symbol level. The passive time-reversal can suppress the inter-symbol interference caused by multipath. Lake experiment results show that the proposed method has no bit transmitting error at 1.8 km range; After adding the noise collected at the test, the BER is at 10-4 when SNR is -8 dB with the 5/4 shift units time error.