The invention discloses a lithium sulfide series solid electrolyte material added with lithium tin alloy and a preparation method thereof. The preparation method comprises the following steps that (1) under the atmosphere protection condition, lithium sulfide, phosphorus sulfide, lithium tin alloy powder and sulfur are weighed according to a mol ratio of (2.5 to 4.0):(0.5 to 1.0):(0.02 to 0.1):(0.01 to 0.05); the materials are uniformly mixed to obtain an amorphous lithium sulfur phosphorus and tin mixture; (2) the amorphous lithium sulfur phosphorus and tin mixture is sealed under the atmosphere protection condition; then, the temperature is raised to 120 to 260 DEG C under the vacuum or atmosphere protection condition; heat treatment is performed; the lithium sulfide series solid electrolyte material is obtained. The concentration of migratable lithium irons in the lithium sulfide series solid electrolyte is improved through adding the high-lithium-content lithium tin alloy capable of easily forming the amorphous state, so that the lithium ion conductivity is improved.