Abstract: In the millimeter-wave band, due to the large path loss, beamforming is usually used to increase the link gain to establish a stable connection. Digital beamforming, which is typically used in the low frequency band, is a power consumptive and costly solution due to the configuration of one RF link per antenna. Using phased array architecture to achieve RF beamforming is a compromise choice. In this paper, we study the beam steering performance loss of finite quantization precision phase shifter in millimeter wave channel. The theoretical relationship between the array gain loss and the error variance of the phase shifter is deduced first, and the results are verified by Monte Carlo simulation. It can be seen from the simulation that the gain loss of the phase array can be reduced by increasing the phase shifter accuracy. In a typical application, a 4-bit phase shifter provides sufficient gain to achieve planar array beam steering.