A method for allocating downlink transmission power of coordinated transmission devices in coordinated multi-point transmission system includes the coordinated transmission devices receiving feedback of channel state information of sub-channels occupied by a terminal from the terminal. The channel state information includes channel phase information and channel amplitude information. Each of the coordinated transmission devices adjusts phases of their sending signals over its sub-channels respectively so as to make phase difference of the sending signals, which are received by the terminal from the coordinated transmission devices via a same sub-channel, to be minimal. Each of the coordinated transmission devices determines its power allocation proportions for the sub-channels on the basis of feedback of channel amplitude information from the terminal and the adjusted phases of the sending signals. By adapting coordinate Iterative Water filling Power Allocation Algorithm with low complexity, the capacity of the system is increased.