Abstract: The variation of channel coefficients autocorrelation in a block time is analyzed when the mobile is assumed to travel at different speed and unitary space-time modulation is employed in the communication system. Computer simulation shows the block error rate of unitary space-time modulation increases with the decreasing of autocorrelation of channel coefficients in a block time. Furthermore, the bigger the block is, the worse the block error rate will be. A new method of unitary space-time constellation,which can improve the influence on block error rate by fast fading, is designed based on a systematic method. Simulation experiments show that the block error rate of the new scheme is much lower than that of the old one when the mobile travels at a relatively high speed.