A method, an apparatus, and a system for forming Time Division Duplex (TDD) Multi-Input Multi-Output (MIMO) downlink beams are disclosed herein. The method includes: generating a transmitting beam of a first data stream according to a Maximum Rate Transmission (MRT) criterion; and generating transmitting beams of other data streams except the first data stream, wherein the transmitting beams of other data streams are confined in a null space of the transmitting beam of the first data stream. Through the embodiments of the present invention, the Base Station obtains precise Partial Channel State Information by using only reciprocity without requiring feedback information, and thus the calculation is simplified and the rate of data streams is improved.