Numerically controlled oscillator is usually adopted in current GNSS(Global Navigation Satellite System) frequency domain anti-jamming receiver. Constrained by hardware, numerically controlled oscillator phase truncation is needed. But, whether numerically controlled oscillator phase is truncated reasonably influences anti-jamming performance greatly. Aiming at the problem, its effect on mixing and frequency domain anti-jamming was analyzed theoretically based on local oscillator spur caused by numerically controlled oscillator phase truncation. A theoretical computing module of numerically controlled oscillator look-up table width was brought forward. Thus, carrier noise loss approximated that of receiver with non-truncated phase numerically controlled oscillator. Simulations show that numerically controlled oscillator look-up table width computed by that module is no more than 10bit, when suppressing narrow band interference whose band is above 100kHz and jam-signal-ratio is below 80dBc. When adopting that width, carrier noise loss of receiver with numerically controlled oscillator phase truncation increases 0.6dB at most, compared to that of receiver with non-truncated numerically controlled oscillator phase.