Conventional secret key generation scheme cannot achieve high key generation rate when the channel characteristics change slowly. To solve this problem, a secret key generation model based on the MIMO received signal space is investigated in this paper. In this model, the legitimate transmitter and receiver distill secret keys from the received signal space, and improve the randomness of the received signals by controlling the variance of the transmitted signals, so that the update of the secret key is independent of the time variance of the wireless channel. Then a specific key generation scheme is proposed, in which the legitimate nodes can obtain the same received signal space, from which common vectors can be selected to distill the keys. In addition, an optimal power allocation method to maximize the key rate is presented. The results of the simulation confirm the effectiveness of this scheme.