The amplitude detection method of the Light-Addressable Potential Sensor (LAPS) is susceptible to noise interference, low sensitivity, low precision, low signal-to-noise ratio, and is greatly affected by the modulated light signal, which affects the accuracy of the detection results. Therefore, a LAPS detection method based on orthogonal phase detection is proposed. In this method, the LAPS photocurrent signal is multiplied by two orthogonal signals, and the direct current component is extracted by low-pass filters and divided, so that the phase information of LAPS photocurrent signal can be obtained. Compared with the existing LAPS phase detection method, the proposed method has the advantages of low algorithm complexity and high real-time performance. The influence of the modulated light intensity on the amplitude detection and phase detection of LAPS is studied. The sensitivity, linearity and signal-to-noise ratio of the conventional amplitude detection method and the orthogonal phase detection method of LAPS to the detection of pH are compared. The results show that, compared with the amplitude detection method, the modulated light intensity has less effect on the phase detection of LAPS. When the frequency at 10 kHz, the pH is from 1.68 to 10.01, the sensitivity of the phase detection method is increased by 7 mV/pH, the precision accuracy is increased by 14.9 mpH, the nonlinear error is reduced by 0.003%, the mean square error is reduced by 0.105 1×10^-5, the signal-to-noise ratio is increased by 8.2827. The proposed method is especially suitable for LAPS detection in weak modulated light intensity.