Abstract: Direct-sequence spread spectrum(DSSS) communication possesses low probability of detection and has been widely used in confidential communications.However,it still exists some defects.In conventional DSSS communications,pseudo-noise(PN) sequences,used as spreading code,possess binary and periodic characteristics.These distinct characteristics may lead some important parameters to be estimated accurately and the leakage of information in hostile environments.To solve the problem,we propose the chaotic phase modulation(CPM) sequence alternating the PN sequences.CPM sequence is complex and has constant envelope,which possesses large quantity and good correlation characteristics.It also possesses higher security than PN sequence by overcoming the problem of periodicity and binary value.To improve the data rate,we apply it into the technique of multichannel communication.Simulation results show this scheme's superior bit error ratio(BER) performance,which demonstrates its feasibility in underwater acoustic communications.