A preparation method of a TiO2/MnS composite photocatalytic coating agent comprises the steps that titanium containing salt, manganese containing salt and thioacetamide are selected as raw materials; concentrated nitric acid is adopted as a hydrolysis buffering agent and drop-added to a titanium salt precursor, namely, the titanium containing salt, until the pH value of the mixture solution is equal to or lower than 2; then water is drop-added so as to hydrolyze titanium salt; TiO2 floccule is obtained after water is drop-added and the mixture is stirred for 0.5-1.5 h; the manganese containing salt is mixed with water, the thioacetamide is taken as a sulfur source, the reaction temperature is controlled in the range from 50 DEG C to 70 DEG C, and MnS floccule is obtained after the mixture is stirred and reacted for 0.5-1.5 h; the TiO2/MnS composite photocatalytic coating agent is obtained after the TiO2 floccule and the MnS floccule are centrifugalized, cleaned for three time, mixed and stirred for 2-28 hours. The preparation method of the TiO2/MnS composite photocatalytic coating agent has the advantages that MnS and TiO2 are fully composited, and high dispersion and uniform distribution state are realized.