Abstract: To cope with frequency selective fading in underwater acoustic communication,one kind of adaptive equalizer of MIMO channels with fractionally-spaced decision-feedback was proposed.Using a second order phase-locked loop(PLL) to compensate the carrier phase shift,the adaptive equalizer not only eliminate code disturb but also not bring about the noise gain.Its merit is suitable to serious distortion in underwater acoustic communication.Finally,single-input single-output(SISO) system was compared the multiple-input multiple-output(MIMO) system with constant modulus algorithm by simulation.