The invention discloses a triaxial test device capable of testing the expansion factor of a geotechnical material. The triaxial test device comprises an upper seat, a base and a check ring, wherein the upper seat is arranged above the base; the check ring extending upward is arranged at the upper part of the upper seat; a top cover is fixedly connected with the upper end of the check ring through a bolt; the upper seat, the check ring and the top cover form an oil cavity; a piston moving axially is arranged in the oil cavity; the piston is of a cross shape and comprises a vertically arranged vertical rod and a horizontally arranged transverse rod; the upper end of the vertical rod stretches upward out of the top cover, and the lower end stretches downward out of the upper seat and enters a test space; the oil cavity is divided by the transverse rod into an upper part and a lower part; and an upper oil conveying channel and a lower oil conveying channel parallel to each other are arranged on the side wall of the check ring along the radial direction. The triaxial test device disclosed by the invention has the advantages of simplicity in operation, accurate measurement system and high automation degree. The invention also discloses a triaxial test method capable of testing the expansion factor of the geotechnical material.