Upper single-null H-mode detachment experiments on EAST were carried out with an ITER-like tungsten divertor, and it was found that the detachment threshold of the upper outer divertor was significantly lower under an unfavorable toroidal magnetic field (Bt) than a favorable Bt. In this work, SOLPS-ITER simulations are carried out to study the effects of Bt direction on the detachment threshold to explain the experimental observations. The simulation results are compared with the experiments, showing reasonable agreement with the diagnostics, and the simulation setups and parameters are thus determined. Based on the simulation data, two-point model formatting is applied to the flux tube where detachment occurs. A quantitative comparison highlights the critical roles of the upstream parallel heat flux and the downstream momentum and energy loss factors on the ion saturation current density at the target ( jsat,t). Further analysis of the simulation results indicates that due to the influence of E×B drift on the convective heat flux, combined with the presence of heat transport associated with parallel current, the upstream heat flux is reduced under unfavorable Bt, while in the downstream region the divergence of the E×B drift flux acts as a particle source under unfavorable Bt and as a sink under favorable Bt, leading to increased momentum and energy losses under unfavorable Bt. These factors collectively cause a lower detachment threshold under unfavorable Bt.