Interferometric synthetic aperture sonar (InSAS) is a novel three dimensional underwater mapping system, which is widely used in various fields. Complex image registration is a crucial process in InSAS signal processing. The commonly used method of co-registration is correlation coefficient in complex domain, which has complex computation. Aiming at the low efficiency of the traditional image co-registration method, a novel approach is proposed to calculate correlation coefficient. This method evaluates correlation coefficient using phase information, which avoids complexity of conjugate multiplication. Experiment results indicate that this method cannot only preserve the registration quality, but also improve the computing efficiency.