The traditional method analyzes the anti-jamming biases by using narrow-band model Hypothesis. On the other hand, next-generation GNSS signals using wider bandwidth make the broadband effect more obviously. Therefore, the impact of narrow-band assumptions needs to be re-examined. This research derives the precise model under the case of DBF processing, and the conclusion is that the array response equals to the inner product of spatial steer vector and the Kroneck product of time steer vector and weights of taps. The research also analyzes the array anti-jam performance biases by narrowband assumption under the multiple factors. The results show that the main reason is the distortion of array response. When JSR is 70dBc, the maximum error introduced by narrow-band approximation model is about 5dB. The intenser the jamming, the bigger the error. At the same time, the error introduced by the above precise model is about 0.5dB.