n-Type AgBiSe₂-based compounds are considered as promising high-performance thermoelectric (TE) materials due to the low lattice thermal conductivity. However, their two phase transitions between 300 and 700 K limits their applications. Therefore, it is crucial to obtain AgBiSe₂-based compounds with stable structures and optimized TE properties. In this work, the Pb-free group IV-VI compound SnTe is selected for alloying with AgBiSe₂. Introduction of SnTe not only reduces the cubic phase transition temperature, but also effectively suppresses the reversible phase transition of AgBiSe₂. At room temperature, reduction of the lattice thermal conductivity from 0.76 to 0.51 W·m^-1·K^-1 results from highly disordered distribution of atoms. Furthermore, Nb dopant to replace Ag, significantly improves carrier concentration of AgBiSe₂-based compounds, which promotes the effective mass and increases the electrical conductivity from 77.7 S·cm^-1 to 158.1 S·cm^-1 at room temperature. Meanwhile, the defect scattering at high temperature is enhanced with the increase of impurity point defects, leading to the lattice thermal conductivity reduced. At 700 K, the lattice thermal conductivity is reduced from 0.56 to 0.43 W·m^-1·K^-1, obtaining stable cubic phase compound (Ag_0.98Nb_0.02BiSe₂)_0.75(SnTe)_0.25 with a ZT of 0.32 at 650 K. These results indicate that the (AgBiSe₂)_0.75(SnTe)_0.25 compound is a promising n-type TE compound with low lattice thermal conductivity and a stable cubic structure. Such efforts provide a scheme for the crystal structure regulation of high-performance TE materials with phase transition and promotion of its application.