For the efficient photocatalytic activity of polymeric photocatalysts, it is necessary to construct effective charge-transfer channels as well as to promote charge mobility. Here, KCl/LiCl salt assisted polycondensation is proposed to synthesize polymeric carbon nitride with a controllable molecular structure. It is notable that the cross-linking degree and the building blocks can be controlled by the ratio of KCl/LiCl. Specifically, with an increasing KCl content in the KCl/LiCl salt system, the product gradually changes from a highly cross-linked two-dimensional carbon nitride to an almost non-crosslinked one. This decreasing cross-linking degree is accompanied by a continuous change of the molecular units from triazine to heptazine. The cross-linking can serve as the inter-chain charge-transfer channel whereas more conjugated subunits will improve the charge mobility. By optimizing the KCl content, effective inter-chain charge transfer and high charge mobility can be realized simultaneously. The optimized polymeric carbon nitride exhibits excellent photocatalytic performance for both H₂O₂ and H₂ production. Significantly, the apparent quantum efficiency (AQE) for H₂O₂ production reaches 56.6% at 420 nm, which is much higher than that of most of the reported carbon nitride-based photocatalysts.