Background <p>Ceramic matrix composites (CMCs) are widely used in high-temperature environments, and due to their low shear strength, failure is primarily governed by shear performance. It is imperative to reveal their shear failure mechanism in-situ under high-temperature conditions.</p> Objective <p>The in-situ shear test of CMCs under high-temperature conditions was realized through the improved Iosipescu method.</p> Methods <p>Based on the traditional Iosipescu method, this study proposes an improved small-scale Iosipescu method with fewer parts and without threaded fastening parts. Furthermore, this method can be applied to high-temperature in-situ loading.</p> Results <p>The specimen's stress field and failure mode were obtained via numerical simulation under the improved Iosipescu method. The in-plane shear strength (IPSS) of the 2D-C/SiC composites from room temperature (RT) to 1100&#xa0;°C was tested under atmospheric conditions using the improved Iosipescu method. The results showed that the IPSS of the 2D-C/SiC composites increased as the temperature rose to 900&#xa0;°C and then decreased as the temperature continued to rise. Furthermore, the in-situ shear test of 2D-C/SiC composite materials at 900&#xa0;°C was performed using the improved Iosipescu method. From the analysis of the tomographic images, it can be seen that the specimen had void defects before the load was applied, and as the load increased, composite material damage began to develop along the original defects until the specimen broke and failed. SEM observed the fracture surface of the sample, and the failure modes at different temperatures were obtained, explaining why IPSS changes with temperature.</p> Conclusions <p>The improved Iosipescu method is used to measure the high-temperature in-plane shear properties of CMCs and can enable high-temperature in-situ testing.</p>