The invention relates to a small-sized ultrahigh-temperature mechanical property testing device. The device comprises an electrode group (1), a main fixed support (2), an auxiliary fixed support (3), a sliding support (4), a loading sliding shaft (5), an upper bottom plate (6) and a lower bottom plate (7), wherein the loading sliding shaft (5) is sequentially connected with the main fixed support (2), the sliding support (4) and the auxiliary fixed support (3); the main fixed support (2) and the auxiliary fixed support (3) are fixed at the two ends of the bottom plates; the sliding support (4) is positioned in the middle and can slide freely; the electrode group (1) is fixed on the main fixed support (2) and the sliding support (4) through an electrode fixing shaft (11) and is clamped by a pre-tightening screw; a driving joint (10) is fixed on the sliding support (4) and is used for driving the sliding support (4) to realize loading. According to the device, electric heating is introduced into the mechanical test of an ultrahigh-temperature ceramic material, and a test piece is directly electrified and heated to reach the ultrahigh temperature. The device is small in size and low in implementation cost, and facilitates lab application.