Suppressing jitter noises in a phase locked loop (PLL) is of great importance in order to keep the precise and continuous tracking of global positioning system (GPS) signals characterized by low carrier-to-noise ratio (C/N0). In this article, a PLL model based on Kalman filter theory is designed to process weak carrier signals in GPS software receivers. By correlating the model error noises and the measurement noises, this PLL model is in a position to make correct estimation of the carrier phase differences and Doppler shifts with less overshoots and noise disturbances, and to keep an effective track of PLL for GPS signal. Finally, the feasibility of this Kalman filter based PLL is verified by the simulation experiment.