Designing effective photocatalysts for carbon dioxide reduction reaction (CO2RR) requires a precise understanding of the dynamically photocatalytic mechanism under real conditions (e.g. solvent, light field). Herein, we coupled ab-initio non-adiabatic molecular dynamics (NAMD) simulation and density functional theory (DFT) calculation to theoretically reveal the detailed dynamic process for C-C Coupling under light field on Co-supported monolayer black phosphorus (BP) catalyst Co@BP. Specially, Co@BP features excellent property for photocatalyst: high stability, long-lived photogenerated carriers and stronger reducing ability. Thermodynamically, it shown that a dramatic difference in catalytic properties with 99.99% selectivity for HCOOH in solvent-free condition (Ea = 0.29 eV) and 99.34% selectivity for CH2CH2 in solution (Ea = 0.33 eV). Slow-growth based MD simulation results uncover that mixed solvent of water/acetonitrile (H2O/ACN) is beneficial for the formation of C2+ product and the optimal ratio H2O/ACN (1:9) mixture solvent for the conversion of CO2 to CH2CH2. Under light irradiation, we found that multichannel photons enable adsorbed CHO* to couple with CHO species diffusing from nearby active sites, further forming C2 intermediates in solution. This work highlights the importance of the reaction medium on the photogenerative carrier dynamics and offers a strategy to regulate product selectivity in photocatalytic CO2 conversion.