Ag-Pb-Sn-Te is an important material system for thermoelectric applications. Various compounds in this system, such as Ag2Te, PbTe, SnTe, (Pb,Sn)Te, are promising thermoelectric materials. There are no phase equilibria information of the Ag-Pb-Sn-Te quaternary system, and limited phase equilibria literatures of the four ternary subsystems, Ag-Pb-Sn, Ag-Pb-Te, Ag-Sn-Te and Pb-Sn-Te. Phase diagrams of Ag-Pb-Te and Pb-Sn-Te are adopted from literatures. Experimental measurements are carried out to determine phase equilibria of Ag-Pb-Sn, Ag-Sn-Te and Ag-Pb-Sn-Te at 350℃. One ternary compound, AgSnTe2, is observed. No other ternary compounds and quaternary compounds are found. PbTe and SnTe form a continuous solid solution (Pb,Sn)Te in the Pb-Sn-Te system. There are two tie-triangles, Ag+ζ-Ag4Sn+Liquid and ζ-Ag4Sn+Liquid+ε-Ag3Sn, in the Ag-Pb-Sn isothermal sections at 350℃. There are eight tri-angles in the Ag-Sn-Te system. The isothermal tetrahedron of the Ag-Pb-Sn-Te quaternary system at 350℃ is proposed based on the literature results and those determined in this study.