Most of the classic wind-driven circulation theories based on the Sverdrup balance have neglected the profound influence of eddy mixing on the large-scale distribution of the potential vorticity (PV), thus failing to explain some prominent features of the observed circulation. In this study, using a series of numerical experiments based on the MITgcm, we diagnose the PV balance to quantify the effect of eddy mixing on the subtropical gyre. Four grid-spacings of 1, 1/3.2, 1/10, and 1/32 degrees are selected to compare the structure of the upper-ocean circulation. In the 1° grid case, the structure of the thermocline is as predicted by the Sverdrup dynamics, with its maximum depth located in the subtropical interior where the wind stress curl is strongest. With increasing resolution, however, this maximum depth is displaced toward the subtropical front, which more closely resembles the observed thermocline. From 1° to 1/32°, the enhanced eddy mixing tends to homogenize the macroscopic PV in the subtropical gyre and reduces the latitudinal PV range to about 25% of the non-eddy solution; and the region where the Sverdrup balance holds is relegated to isolated patches, with its area reduced by about 60%. Furthermore, sensitivity experiments show that the observed thermocline structure is well reproduced in eddy-resolving runs, indicating that the PV mixing provides a better explanation of the subtropical circulation than the Sverdrup dynamics. Our results suggest that the Sverdrup relationship should be treated carefully in the eddy-rich region, even in the subtropical interior.