The adsorption properties of Cu, Ag, Zn, and Cd-modified SnP₃ monolayers for H₂S have been studied using density functional theory (DFT). Based on phonon spectrum calculations, a structurally stable intrinsic SnP₃ monolayer was obtained, based on which four metal-modified SnP₃ monolayers were constructed, and the band gaps of the modified SnP₃ monolayers were significantly reduced. The adsorption capacity of Cu, Zn-modified SnP₃ was better than that of Ag, Cd-modified SnP₃. The adsorption energies of Cu-modified SnP₃ and Zn-modified SnP₃ for H₂S were −0.749 eV and −0.639 eV, respectively. In addition, Cu-modified SnP₃ exhibited chemisorption for H₂S, while Zn-modified SnP₃ exhibited strong physisorption, indicating that it can be used as a sensor substrate. Co-adsorption studies showed that ambient gases such as N₂, O₂, and H₂O had little effect on H₂S. The band gap change rate of Zn-modified SnP₃ after adsorption of H₂S was as high as −28.52%. Recovery time studies based on Zn-modified SnP₃ showed that the desorption time of H₂S was 0.064 s at 298 K. Therefore, Zn-modified SnP₃ can be used as a promising sensor substrate for H₂S due to its good selectivity, sensitivity, and fast recovery time.