Radar Cross Section (RCS) serves as the core physical quantity characterizing the electromagnetic scattering properties of a target, and its measurement accuracy directly underpins the reliability of equipment R&D, design, and test evaluation. Phased array radar, leveraging advantages such as beam agility and multi-mode switching, is progressively emerging as the pivotal equipment for modern RCS measurement. However, the electronic scanning mechanism introduces unique error sources associated with scanning states and signal timing, which exert a significant impact on measurement accuracy. In this paper, by analyzing the relevant parameters and beam deflection-induced uncertainties in phased array radar spot-mode measurement, an RCS measurement method based on zero-Doppler background clutter extraction is proposed. Combined with simulation analysis, the contribution weights of various factors are clarified. This study systematically investigates the influence patterns of each factor on the RCS measurement accuracy of phased array radar, defines the contribution weights of key factors, and provides critical simulation support and theoretical basis for enhancing measurement accuracy and scenario adaptability.