In present paper, the structural, mechanical, and electronic properties of the Sn2P2S6 compound under different pressures by the density functional methods in the generalized gradient approximation have been examined in the ferroelectric (Pc) and paraelectric (P2_1/c) phases. The lattice parameters, mechanical properties, electronic bands structures and partial density of states for both phases are presented and analyzed. The nonlinear optical properties and electro-optic effects of Sn2P2S6-Pc have been studied by the density functional theory in the local density approximation. Our structural estimation and some other results are in agreement with the available experimental and theoretical data. We present calculations of the frequency-dependent complex dielectric function (ω) and the second harmonic generation response coefficient χ(2) (−2ω, ω, ω) over a large frequency range. The electronic linear electro-optic susceptibility χ(2) (−ω, ω, 0) is also evaluated below the band gap. These results are based on a series of the LDA calculation. The results for χ(2) (−ω, ω, 0) are in agreement with the experiment below the band gap and those for χ(2) (−ω, ω, 0) are compared with the experimental data where available.