Constructing heterostructures by combining COFs and TMD is a new strategy to design efficient photocatalysts for CO2 reduction reaction (CO2RR) due to their good stability, tunable band gaps and efficient charge separation. Based on the synthesis of completely novel C4N-COF in our previous reported work, a new C4N/MoS2 heterostructure was constructed and then the related structural, electronic and optical properties were also studied using first principle calculations. The interlayer coupling effect and charge transfer between the C4N and MoS2 layer are systematically illuminated. The reduced band gap of the C4N/MoS2 heterostructure is beneficial to absorb more visible light. For the formation of type-Ⅱ band alignment, a built-in electric field appears which separates the photogenerated electrons and holes into different layers efficiently and produces redox active sites. The band alignment of the heterostructure ensures its photocatalytic activities of the whole CO2 reduction reaction. Furthermore, the charge density difference and charge carrier mobility confirm the existence of the built-in electric field at the interface of the C4N/MoS2 heterostructure directly. Finally, the high optical absorption indicates it is an efficient visible light harvesting photocatalyst. Therefore, this work could provide strong insights into the intal mechanism and high photocatalytic activity of the C4N/MoS2 heterostructure and offer guiding of designing and synthesizing COF/TMD heterostructure photocatalysts.