In the Western Pacific Ocean (WPO), the North Equatorial Current (NEC), Mindanao Current (MC) and Kuroshio Current (KC) form a circulation system which is known as NMK. The NMK plays an important role in the heat, salt, energy and biogeochemical substance transports between the WPO and the adjacent China Sea (CS). NMK is crucial in determining the ocean circulation and marine ecosystem of the CS. However, the process and dynamics in the NMK and its linkage with corresponding processes in the CS remain largely unknown due to lacking of observational data and geophysical understanding. By combining remote sensing data, in situ ARGO measurements with results from the China Sea Multi-scale Ocean Modeling System (CMOMS), we conducted comprehensive studies on the spatiotemporal variability of the NMK and its underlying intrinsic physical process and controlling mechanism. The water masses, total transports and structures of the NEC, MC and KC exhibited prominent spatiotemporal variabilities. It was found that the temporal variation of the NEC flux center controlled the NEC bifurcation latitude (NECBL) in inter-annual time scale, while the wind stress curl in the region determined the seasonal variation of the NECBL. Variability with ~90-day period was found in the KC both in the CMOMS and in the observations, which can be explained by the westward propagation of ocean eddies. Below the NEC, alternative jet-core structure was detected within the NEC Undercurrent region. There also existed the Mindanao Undercurrent (MUC) and Luzon Undercurrent (LUC) in MC and KC, respectively. Different from the previous studies, the LUC was found to be originated from the reverse of the KC due to island effect near Luzon Strait. Through analyses of momentum and depth-integrated vorticity balances, we found that the intensity of the NEC, KC and MC were mainly controlled by the cross-stream pressure gradient force (PGF) due to the quasi-geostrophic balance, and frequently modulated by ageostrophic effect arising from wind forcing in winter and from nonlinear advection in summer. The origin of the cross-stream PGF is found to be composed of baroclinic plus sloping effect and bottom PGF. The analyses also revealed that the south-north shifting of the NEC was controlled by the along-stream PGF variation that was formed mainly by beta effect and partly by Modified Joint Effect Baroclinicity and Relief and wind induced vertical viscosity. The spatiotemporal variability of NEC, KC and MC was dynamically linked. This study revealed the three-dimensional, time-dependent variability of the NMK and provided new understandings of its underlying forcing mechanism.