In coherent detection of a high-speed optical communication system, blind equalization is a common technique to recover transmitted symbols, and one of the most widely used algorithms is constant-modulus algorithm (CMA). But on one hand, it can not converge fast and precisely enough; on the other hand, it can be caught in singularity problem while operating on a polarization division multiplexed system. A modified CMA, which is adaptive step-size CMA, is proposed. The performance of the proposed algorithm is numerically tested in a 112 Gb/s polarization division multiplexed 16 quadrature amplitude modulation (QAM) optical coherent system. Compared with conventional CMA, the convergence rate of the modified CMA is 20 times faster, and the variance of the error function is 0.7 dB lower. Besides, the modified CMA can overcome the singularity problem quite well. Without considering the singularity problem, the optical signal noise rate (OSNR) penalty is 15 dB lower than that of conventional CMA.