A system for selecting a transmission mode under the multi-input multi-output (MIMO) architecture based on a scheduling number and a method thereof are provided. A modulation coding of the spatial diversity (SD) mode and the spatial multiplexing (SM) mode according to channel state information sent from client ends is determined, respectively. The SD mode or the SM mode is selected in accordance with a scheduling number of using the SD mode and a scheduling number of using the SM mode, respectively. The transmission mode is selected based on the spectrum utilization and the scheduling rate, which achieves improving the spectrum utilization on the premise of the scheduling rate.