A new metal-mixed chalcogenide Ba7AgGa5S15 was successfully synthesized by high temperature flux spontaneously crystallization method. The compound crystallizes in the asymmetric space group P31c(No.159) with the cell parameters of a=0.964 53 (10) nm, c=1.805 9 (4) nm and Z=2. The structure of Ba7AgGa5S15 is regarded as an 18-member tunnel-contained three-dimensional (3D) framework formed by the［Ga4S10］ T2 supertetrahedra and the［AgS4］ tetrahedra by sharing vertexes. Isolated［Ga(2)S4］ tetrahedra locate inside the tunnels and the Ba2+ cations insert in the space of the framework. The optical properties of the title compound were systematically studied based on first-principles calculations, which includes the electronic structure, density of states, birefringence, the second order nonlinear optical (NLO) coefficients and the second-harmonic generation (SHG) density. The results show that Ba7AgGa5S15 has a direct band gap of 3.76 eV, which is mainly determined by the S 3p, Ba 5d and Ga 4s orbitals. The NLO coefficient in the d33 direction is ~0.4 times that of AgGaS2(AGS), and the main contribution of SHG is from [AgS4] and [GaS4] tetrahedrons. This work indicates that introducing alkali earth metal Ba2+ into the Ag-Ga-S system could effectively enlarge the band gap of the formed Ba7AgGa5S15 compared with classical AGS compound, which is helpful in producing higher laser damage threshold (LDT).