Abstract: Due to the dual constraints of insufficient strength of fixture materials in high-temperature environments and the incompatibility of sample configurations, the selection of fixture materials and the structural design for the high-temperature mechanical property testing of ceramic matrix composites (CMCs) have become a hot topic. Si₃N₄ ceramics with excellent high-temperature resistance, mechanical properties and phase structure stability under high-temperature conditions were selected as the fixture material. A high-temperature fixture suitable for the dovetail-shaped plate-like CMCs was designed and fabricated, and its feasibility in the mechanical property testing of CMCs at 1 000 ℃ was verified. The thermal stress distribution and stress state of Si₃N₄ ceramic fixture and silicon carbide composite were analyzed by Abaqus finite element analysis software. The results showed that the stress concentration in the fixture weak area was significantly reduced by combining the innovative dovetail-shaped sample clamping structure with the chamfered stress release design. Moreover, the fixture exhibited excellent load conduction and failure resistance property in the high-temperature service environment. The test results of high-temperature tensile and high-temperature fatigue properties of CMCs at 1 000 ℃ indicated that this fixture exhibited stable mechanical response and durability at high-temperatures.