Abstract: GNSS (global navigation satellite systems) time uses atomic clock as the time standard while GNSS receiver uses crystal oscillator. The stability of atomic clock is several times of magnitudes better than crystal oscillator. Therefore, the intersystem time offset between different GNSS is more stable than receiver clock bias. It will benefit the positioning results if we can fully use that prior information. The purpose of this contribution is to analyze how to use that prior information properly and examine the influence on it. We deduce two positioning model based on different clock bias estimation method and introduce two parameter estimation algorithm:least square (LS) and extended Kalman filter (EKF). Static and dynamic tests were carried out and some conclusions are made:the LS cannot improve the positioning performance with the prior information of that the intersystem time offset between different GNSS is more stable than receiver clock bias. Static test results show the EKF itself has some kind of noise suppression. The noise is less in case we use the "stable" prior information. Dynamic test shows divergence of EKF can be suppressed by using the "stable" prior information.