Two-dimensional materials with an intrinsic internal electric field possess promising potential to improve the photocatalytic water-splitting performance. However, the construction of the internal electric field is still a great challenge, which requires that the material itself should exhibit spontaneous symmetry breaking with intrinsic polarization. Herein, we propose using a general intercalation approach to introduce a spontaneous polarization electric field into a single-layer lattice by constructing spatially asymmetric configurations. Taking septuple-atomic-layer MoSiN as a model material, following the above design principle, four promising MSiNY (M = Mo, W; Y = P, As) monolayers are theoretically identified, exhibiting excellent stabilities, suitability and low reaction barriers for overall water splitting. Importantly, the intrinsic internal electric field of MoSiNY promotes the charge-carrier separation and improves the light absorption capacity simultaneously, thus enabling a high solar-to-hydrogen efficiency of 29.84-32.93%. Meanwhile, the carrier-transfer dynamic processes are explored, which demonstrate that MSiNP monolayers possess a low electron-hole recombination rate, suggesting their highly efficient photocatalysis. This study opens up an avenue to rationally engineer the internal electric field and contributes to enhancing the photocatalytic efficiency.