Spatial Quadrature Amplitude Modulation (SQAM) modulates the spatial phase and intensity of the signal beam, and attracts attention as a signal modulation method for large capacity holographic data storage. We have proposed the phase-shift embedding technique to demodulate the SQAM signal with single shot interferogram capturing. In this paper, we investigate the improvement of the accuracy of SQAM signal demodulation with the phase-shift embedding method by simulation and optical experiment.