By combine the crystal structure prediction methods based on particle swarm optimization algorithm with the first-principles calculations, we have systematically studied the crystal structures and the electronic properties of bulk SnS, monolayer SnS, bilayer SnS and trilayer SnS. The results show that due to the influence of the intermolecular forces between the layers, the most stable structures of monolayer, bilayer and trilayer SnS at ambient pressure are all orthogonal structures, but they are different from the bulk SnS (Pnma). The space group of monolayer SnS is C2/m, both bilayer SnS and trilayer SnS are P4mm. The calculation results of electronic properties show that the number of layers will effectively modify the electronic structures. For example, the band gaps of monolayer, bilayer and trilayer SnS are 1. 35 eV,0. 24 eV, and 0. 17 eV respectively; There are polar covalent bonds between Sn and S atoms in all structures. The electrons are mainly localized around S atoms. With the number of layers increasing, the amount of charge transferred from Sn to S atoms decreases and the degree of charge localization decreases, and the bonding is strengthened. The research will provide theoretical guidance for the synthesis of few-layer SnS and its application in the field of solar cells and thermoelectrics. [ABSTRACT FROM AUTHOR]