Abstract: The frequency source in Global Positioning System (GPS) receiver has a great many influence on the whole performance of the receiver. Based on the analysis of error characteristics and performance effect of the mostly used crystal oscillator, a method to simulate the frequency error of crystal oscillator is proposed. The major idea is to determine the Allan variance parameter of each main stochastic error component at the given sampling interval at first, then to deduct a set of corresponding frequency error series and superpose them together. The computer numerical simulation approach and the experimental results are given. This research provides related both theoretical reference and practical design for the development of an integrated design, test and evaluation platform such as a GPS signal simulator.