Differential information broadcast by the Ground-Based Augmentation System (GBAS) cannot correct for uncorrelated errors between the GBAS ground subsystems and airborne subsystems, e.g, multipath errors, making it the largest error source of GBAS in the nominal environment. Before the GBAS reference station is formally deployed, the initial station site must be evaluated quantitatively for the multipath effect to verify that the current site location meets the installation requirements. The multipath effect assessment of an actual installed GBAS reference station is performed based on the code minus carrier (CMC) methodology. The results show that the GBAS reference station deployment environment can meet the multipath error requirements. Simultaneously, the carrier smoothing algorithm can effectively suppress the multipath effect of the site, the algorithm becomes much more effective with the smoothing time constant increasing. Under the current installation conditions, the results of the multipath error-index term of the GBAS reference station were found satisfying the requirements of the GAD-B3 class accuracy level.