An IQ demodulation model for dual-balanced heterodyne detection is established, by rotating λ/4 wave plate to change the phase difference between the IQ signals. Signal-to-noise ratio (SNR) of balanced heterodyne detection is simulated with local excess intensity noise, shot noise and thermal noise. Simulation results show that signal-to-noise ratio of conventional heterodyne detection is higher than that of balanced heterodyne detection, when the beam splitter coefficient adjusted by rotating λ/2 wave plate is less than 0.272 or much than 0.728. Whereas, signal-to-noise ratio of conventional heterodyne detection system is less than that of balanced detection system. Finally, 2 μm dual-balanced heterodyne detection system is proposed, in which the signal-to-noise ratio is 10 dB higher than that of conventional heterodyne system with 0.5 beam-splitter coefficient. This result verify feasibility and advantage of the balanced heterodyne detection system for detecting weak signal.