In this paper, we show the method to reduce complexity of demodulation in passive Multiple-Input Multiple-Output (MIMO) scheme, which has been proposed to improve transmission rate in passive Radio Frequency Identification (RFID). In order to reduce complexity of Maximum Likelihood Detection (MLD), analog signal processing is used to decouple the streams by using beam-forming at receiver. On the basis of the simulation, it is shown the proposed demodulation method can improve the complexity of MLD with slight deterioration of BER. Also, it is shown the proposed method is effective especially in reducing complexity of passive MIMO demodulation. This advantage becomes remarkable in proportion to the number of tag antennas.