The deception jamming of GPS is realized through launching pseudo GPS signal to replace the real GPS signal received in the target receiver. Through providing false navigation message and false pseudo-range information in the pseudo GPS signal, it can affect the positioning of the target receiver and make it get error position. This paper analyses how to replace real GPS signal in the target receiver tracking loop and how to set the parameters in the pseudo GPS signal for implementing GPS spoofing successfully. Matlab simulation is made for verification.