The outdoor mean radiant temperature (T_mrt) is largely influenced by the direct solar radiation incident on the human body, mainly affected by the projected area factor (f_p). The f_p has been calculated by different methods and compared by some studies. However, how the f_p quantitatively affects outdoor thermal comfort has rarely been studied. Therefore, this study established sitting, standing, and walking human models to calculate the f_p in different solar positions through Monte Carlo ray-tracing technology. A case study has been taken in Shanghai to analyze the effect of the f_p on outdoor thermal comfort. The Fanger, Jendritzky, and Underwood models' applicability in Shanghai was compared. The f_p of sitting, standing, and walking postures have been obtained, respectively. The results show that more than 0.05 f_p difference may cause half of the thermal grade deviation for outdoor thermal comfort evaluation. The f_p is more sensitive to solar altitude (β) in three postures than solar azimuth (α). The f_p difference from different postures can cause more than half of thermal grade deviation. Fanger's model for sitting posture may cause one thermal grade deviation; Jendritzky's model has acceptable performance for three postures; Underwood's model for sitting and walking postures may result in nearly one thermal grade deviation.