Transitional metals are widely used as co‐catalysts boosting photocatalytic H2 production. However, metal‐based co‐catalysts suffer from high cost, limited abundance and detrimental environment impact. To date, metal‐free co‐catalyst is rarely reported. Here we for the first time utilized density functional calculations to guide the application of phosphorene as a high‐efficiency metal‐free co‐catalyst for CdS, Zn0.8Cd0.2S or ZnS. Particularly, phosphorene modified CdS shows a high apparent quantum yield of 34.7 % at 420 nm. This outstanding activity arises from the strong electronic coupling between phosphorene and CdS, as well as the favorable band structure, high charge mobility and massive active sites of phosphorene, supported by computations and advanced characterizations, for example, synchrotron‐based X‐ray absorption near edge spectroscopy. This work brings new opportunities to prepare highly‐active, cheap and green photocatalysts. Density functional calculations were used to direct the design of phosphorene as a metal‐free co‐catalyst promoting photocatalytic H2 production in metal sulfide photocatalyst systems. The enhanced photocatalytic performance arises from the pronounced electronic coupling between metal sulfides and phosphorene, together with its advantageous band structure and excellent charge carrier mobility.