Two transmit antennas placed symmetrically for a linear cell downlink distributed antenna system with Alamouti space-time block coding over Rayleigh fading channels. For mobile station located in different places in the cell, which mode should be selected by base station from one transmit antenna or two transmit antennas? Aiming to solve this problem, a transmit mode selection scheme is developed based on received power. In the proposed scheme, the difference of received power from different transmit mode is compared with the preset threshold for deciding the mode to minimize bit error probability. Numerical and simulation results indicate that, under the condition of distance between transmit antennas 500m and standard deviation of shadowing 8dB, when the distance between mobile station and transmit antenna is less than 80m, the mode with one transmit antenna is selected, and in this area, at the area averaged bit error probability of 10-2, the proposed scheme gains transmit power about 1.5dBm relative to the distributed Alamouti scheme with two antennas.