We investigated performance of an open boundary condition for tidally and subtidally (TST-OBC) forced shelf circulation over a limited-area, one-way downscaling numerical modeling system in the northern South China Sea (NSCS). The circulation in this downscaling modeling system was driven by both subtidal and amplified tidal forces with notable spatial and temporal variations. The TST-OBC numerically and physically well accommodated the circulations driven not only by the regional tides, subtidal forces and external forcing from its upscale solution, but also by the internally generated disturbances. The model well captured the observed characteristics of circulations under complex dynamic environment, as a result of the numerical and physical advantages in TST-OBC, such as separation of fast tidal and slow subtidal mode, adoptions of active (rather than passive) and dual-wave transmitting scheme, consistent treatments of barotropic, baroclinic circulations, and thermodynamic variables. The results greatly contrasted from those obtained from the frequently adapted Flather-type OBC in the tidally forced open boundary, which over-strongly restored internal to external solutions with creation of a notable accumulation of spurious disturbances along open boundaries. Disturbances travelling across the computational domain and arriving open boundaries were unrealistically swapped by FLA-OBC, and a sensible communication between submodels in the downscaling modeling system was prevented without adopting the advantages of the TST-OBC.