Abstract: Objectives To determine and obtain the real-time system time offset between different global navigation satellite system time references (GNSS time offset) in real time is the premise and basis of multi-GNSS joint navigation, positioning and timing. In order to improve the accuracy of GNSS time offset monitoring and overcome the shortcomings of existing time difference monitoring models such as incomplete delay correction, we propose a GNSS time offset monitoring method (called the modified PPS method) based on a standard BeiDou Time (BDT) one pulse per second (1 PPS) signal measurement, and designs an evaluation and analysis strategy for the corresponding time offset data series. Methods Firstly, the hardware platform of time offset monitoring is built by assembling several high-performance equipment and the time offset monitoring link is accurately calibrated and corrected. Then, the system errors such as time delay are optimized through the closed monitoring of BDT. Finally, the time offset data is processed based on the designed data processing method. Thus, real-time and high-precision GNSS time offset monitoring is realized. In addition, an evaluation strategy that can consider both time-frequency characteristics and precision characteristics is constructed to comprehensively analyze the time offset monitoring results. Specifically, the accuracy of the time offset results and the variation between different GNSS time systems are analyzed by calculating the accuracy, drift rate, short-term stability (using modified Hadamard variance) of the time offset data series and the uncertainty of the corresponding fitting residual. Results The results of time offset monitoring and its corresponding analysis show that the uncertainty of time offset measurement between BDT and GPST/GST/GLOT obtained by the new method is better than 3 nanoseconds, and the accuracy of the new method is 4 times higher than that of the single point positioning method on average. The proposed strategy can consider the time-frequency characteristics and precision characteristics of the time offset results to achieve a more comprehensive system evaluation and analysis. Conclusions The modified PPS method can carry out reasonable and accurate calibration and correction of the time offset monitoring link, while the closed monitoring based on BDT can refine the systematic errors of the model and the designed data processing method can process the time offset data more accurately and reasonably. As a result, the method can realize real-time and high precision monitoring of GNSS time offset. Furthermore, the time offset results are evaluated and analyzed scientifically and effectively from different angles based on the proposed strategy. BDT keeps good consistency with other GNSS time references.