To address the poor minimum detection performance and low precision of traditional non-modulated turbidity detection technology in low turbidity measurements, a turbidity detection technology based on orthogonal phase-sensitive demodulation is proposed. Based on the principle of turbidity detection, the propagation mechanism of scattered light in water is analyzed, and the response characteristic relationship between turbidity and scattered light intensity is established. To extract weak photoelectric signals, an orthogonal phase-sensitive demodulation circuit consisting of dual-channel analog switches is used. The characteristics of frequency selection and phase identification of the orthogonal phase-sensitive demodulation principle are derived in detail. By selecting the signal spectrum and correcting phase deviation influences, the suppression of noise that is neither same-frequency nor same-phase is achieved. Therefore, the signal-to-noise ratio of the system is enhanced. A turbidity sensor is developed, the corresponding test apparatus is constructed, and the performance is evaluated. Experimental results show that, within a 0 ~ 5 NTU measurement range, the turbidity sensor has good linearity, accuracy, and stability. The limit of detection can reach as low as 0. 004 9 NTU. This method significantly improves the minimum detection performance and measurement accuracy and methodologically avoids the impact of DC drift noise and broadband noise on turbidity measurements at low signal-to-noise ratios.