The phase detection method of Light-addressable potentiometric sensor (LAPS) can realize accurate detection under the weak light intensity. Now available phase detection method has high algorithm complexity and is difficult to achieve. Aiming at this practical problem, a phase detection method based on orthogonal detection is proposed. This method takes the modulation signal of light source and its orthogonal signal as the reference, multiply with the LAPS signal, respectively. After that extracts the dc component through low-pass filtering and divide. Then could be able to obtain the phase information of the LAPS signal. In this paper, the sensitivity and linearity of H+ detection and the influence of the modulated light intensity on the LAPS detection system are analyzed by orthogonal detection phase detection method. The results show that at the frequency of 4KHz and the detection pH units of 4.00~10.01, with a sensitivity of 60.08 mV/pH and non-linear error of 0.013%, and the modulated light intensity has slight effect on LAPS phase detection. This method has low complexity, high accuracy and excellent sensitivity and linearity for LAPS detection.