Two-dimensional (2D) transition metal dichalcogenides (TMDs) and their van der Waals (vdW) heterostructures provide a compelling platform for advanced electronic and optoelectronic applications by utilizing their unique quantum confinement and strong light-matter interaction properties. The performance of such devices is fundamentally governed by interface charge transfer dynamics. Although recent experiments have shown that inserting insulating layers can effectively regulate these dynamics, there is still no predictive theoretical model to quantify how inserting insulator affects the probability of charge tunneling. Moreover, the mechanism details of how layer-thickness changes regulate the potential barrier height and width to adjust charge transfer have not been fully resolved. Here, we establish a theoretical framework by combining atomic-bond-relaxation theory with the quantum tunneling model, to systematically investigate how an h-BN dictates the interlayer barrier landscape and tunneling probabilities in WS₂/h-BN/MoS₂ heterostructures. We find that increasing h-BN thickness, which shifts its conduction band minimum downward and valence band maximum upward, reduces the heterostructures barrier height while increasing its effective width. Surprisingly, for a fixed h-BN thickness, the barrier height in the WS₂/h-BN/MoS₂ stack increases with the increase of TMD constituent thickness, but remains lower than that of monolayer-WS₂/h-BN/monolayer-MoS₂ heterostructure with the same h-BN thickness. This elevated barrier in the monolayer configuration is attributed to the large exciton binding energies of monolayer WS₂ and MoS₂. Further analysis reveals that when the thickness of h-BN remains constant, the charge tunneling probabilities for electrons and holes are primarily controlled by the barrier height and display non-monotonic dependence on TMD thickness, initially rising then falling. The decrease in the monolayer-WS₂/h-BN/monolayer-MoS₂ case is related to the barrier enhancement caused by strong excitonic effects. In contrast, with TMD thickness fixed, tunneling probabilities decay sharply and exponentially as h-BN thickness increases, with extracted attenuation constants of 0.77 Å^–1 for electrons and 0.74 Å^–1 for holes. This is a direct consequence of the widening tunnel barrier suppressing coherent interlayer transport. Our results provide a foundational model for understanding and engineering charge transfer dynamics through inserting insulating layers in vdW heterostructures, and offer critical insights for designing customized optoelectronic interfaces.