In response to the demand for precise birefringence measurements, this study investigates the utilization of double photoelastic modulation for birefringence measurements. The primary focus lies in addressing challenges related to online, in-situ calibration of the photoelastic modulator (PEM) and data processing within the measurement setup. Digital phase-locking technology is employed to facilitate the assessment of multiple modulation signal frequency components. The amplitude ratio of each frequency modulation signal is employed for online, in-situ calibration of PEM phase amplitude. Furthermore, leveraging this amplitude ratio enables the determination of birefringent retardance and fast-axis azimuthal angle measurements. An experimental setup was constructed and experimentation conducted using Solier’s Barbierne compensator as a standard sample. Results demonstrate that the proposed approach enables real-time online, in-situ calibration of PEM phase amplitude, facilitating high-precision, high-sensitivity, and rapid birefringence measurements. The relative error remains below 0.60%, and standard deviation doesn’t exceed 0.049 nm within a 200 ms timeframe. This method exhibits promising prospects for the real-time detection of birefringence in optical materials and components, offering significant implications for practical applications in online birefringence detection.