This paper presents a method of reducing excitation error in sidelobe canceller without increasing the resolution of the digital phase shifters and the digital attenuators. In general sidelobe canceller, the null direction is shifted because of the excitation error (quantization error and random error, etc.) and the suppression capability degrades. The proposed method can alleviate the influence of the excitation error by vector composition of some quantized excitation weights. Computer simulation results show that the output signal to interference and noise power ratio (SINR) using the proposed method can improve greatly in comparison with that using conventional quantized excitation weight.