The invention discloses a uniaxial tension test outward extension type deformation measurement device in an ultrahigh-temperature environment. The device comprises a high-temperature furnace (1), an environmental chamber small box (2), an upper loading pull rod (3), a lower loading pull rod (4), a ring sleeve (5), a clamp (6), an upper clamping ring (8), a lower clamping ring (9), a first connecting pin (11), a stretching rod (12), an upper bracket (13), a lower bracket (14), a bolt (15), a rolling bearing (16), a second connecting pin (17), a guide slot (18), a locking spring (19), a linear variable differential transformer (20), a circumference constraint device (21) and a third connecting pin (22). By the adoption of an outward extension type design, a displacement measurement part is transferred to a normal temperature area outside the high-temperature furnace from an ultrahigh-temperature area in the high-temperature furnace, the quantitative research of the ultrahigh-temperature extension test is realized, and the measurement precision is improved; instrument parts in the ultrahigh-temperature area are made of ultrahigh temperature resistant materials; moreover, the mechanical properties of the ultrahigh-temperature materials are tested and researched, the operation is simple, and the feasibility is high.