Interference and multipath were the two dominant factors that affect the guidance and positioning performance of GNSS (Global Navigation Satellite System). According to the problem of anti-interference, a two-stage anti-jamming algorithm was proposed for GNSS antenna array. In the first stage, the PI (power inversion) algorithm was applied to suppress the interference. Then, the SNR (Signal-to-Noise-Ratio) of GNSS signal was improved by dispreading processing, and the spatial signature vector was estimated. In the second stage, the estimated spatial signature vector was used to weight and sum the output signal of the first stage. The proposed algorithm could null the interference and steer beams in the direction of GNSS signals simultaneously. Simulation results show that the performance of the proposed algorithm is better than PI algorithm, and it is similar to MVDR (Minimum Variance Distortionless Response) algorithm. The proposed algorithm requires no array calibration and aided altitude measurement unit, which has less implementation cost than MVDR algorithm.