The invention provides a method for monitoring the dynamic deflection of a high-speed rail bridge based on a single-frequency GNSS positioning technology. The method includes following steps: S1, a receiver obtains GNSS single-frequency carrier phase observation values of a base station and a bridge monitoring site in real time; S2, a to-be-monitored satellite is selected by employing a factor dual restraint method of E-ADOP and GDOP; S3, a convection layer real-time model is established based on a spherical cap harmonic analysis, and a convection layer delay is resolved; S4, a multi-path real-time model is established, a multi-path error correction value is resolved, and a satellite-earth distance value is updated according to the error correction value; S5, a GNSS single-frequency carrier phase double-difference observation quantity is calculated based on the convection layer delay and the updated satellite-earth distance, and the ambiguity is resolved; and S6, performing back substitution of the ambiguity, calculating a three-dimensional coordinate value of the monitoring point of the bridge in real time, and obtaining a dynamic deflection value of the bridge. According to the method, the operation and the calculation are simple, the dynamic deflection of the high-speed rail bridge can be automatically monitored in real time, measuring data is continuously corrected throughmodeling, and the accuracy of dynamic monitoring of the bridge is improved.