In order to improve the reliability of precise time comparison, this paper takes advantage of long-term stability of Two-Way Satellite Time and Frequency Transfer (TWSTFT) and short-term stability of GPS Precise Point Positioning (PPP). Stability weighting, Vondrák-\vCepek combined filtering, and Kalman filtering methods are applied to fuse the TWSTFT and GPS PPP time comparison between the National Time Service Center (NTSC) and Physikalisch-Technische Bundesanstalt (PTB). The results show that the three fusion methods all improve the diurnal variations in TWSTFT and the discontinuities in GPS PPP solution, and the absolute value of the Double Clock Difference (DCD) between fusion results and PPP solution is kept within the link calibration uncertainty. The 1 d time and frequency stability of the three methods can reach the order of sub-nanosecond and 10^-15. The Vondrák-\vCepek fusion method has the highest stability within 1d, which is suitable for the fusion of time comparison links with requirements for short stability. Stability weighting and Kalman filtering fusion have better fidelity, which are suitable for the fusion of time comparison links that require high accuracy.