The invention discloses a nonstoichiometric ratio Bi-Ag-S series thermoelectric material and a preparation method, and belongs to the technical field of energy materials. The method for preparing the nonstoichiometric ratio Bi-Ag-S material provided by the invention is characterized in that: high-purity metal simple substance Bi, Ag powder and high-purity S powder are used as raw materials and are prepared according to a chemical general formula of Bi2-xAgxS3 and Bi2-xAg3xS3-y (wherein the x is the molar fraction of an Ag component, the value range of the x is more than or equal to 0.001 and less than or equal to 1, the y is the molar fraction of the lacking amount of a sulfur element, and the value range of the y is more than 0 and less than or equal to 1.5); and the raw materials are synthesized into compound powder by adopting a mechanical alloying method, and then the compound powder is sintered through discharge plasma to form blocks. The method can simply, conveniently and accurately prepare the nonstoichiometric ratio Bi-Ag-S series block material, control the thickness of material carriers and the category of main carriers through the nonstoichiometric ratio of the components, and improve the thermoelectric performance.