For a Multiple-Input and Multiple-Output (MIMO) Orthogonal Frequency-Division Multiplexing (OFDM) system with universal space-time codes and correspondent least-squared decoders, this study presents an improved eigenmodes transmission according to the of equivalent channel matrix. With novel eigenmodes, data symbols encoded by space-time codes can be steered to these eigenmodes similar to MIMO wireless communication systems with single-carrier transmission. Moreover, the relations among different code-rate space-time codes, system capacity and number of eigenmodes are respectively evaluated by numerical simulation. According to this, an improved water-filling scheme is proposed for determining the optimal transmit powers for orthogonal eigenmodes. Compared with classical water-filling scheme, it can also obtain larger throughputs via residual power allocation. Results indicates that the improved water-filling scheme can obtain good tradeoff, with comparison to classical water-filling schemes and greedy algorithms, respectively.