For multiple-input multiple-output (MIMO) wireless communication system with minimum mean square error (MMSE) detection, a new scheme of power allocation between pilot and data symbols is investigated under MMSE channel estimation in this paper. First, we propose a novel soft-output MMSE MIMO detector by taking into consideration the channel estimation error. Then, through the application of random matrix theorem, we propose an efficient scheme for power allocation between pilot and data symbols which maximizes the lower bound of postprocessing signal-to-interference-and-noise ratio (SINR) for MIMO systems with equal number of transmitter and receiver antennas. We have proven the existence and uniqueness of the proposed optimal power allocation settings. Furthermore, our analysis shows that the proposed power allocation is also valid and applicable for those MIMO systems with unequal number of transmitter and receiver antennas. Finally, our extensive simulation results have validated this novel power allocation scheme.