Abstract: In-plane shear testing program was designed for mullite fiber reinforced aerogel composites based on the V-notched two-rail shear method. In-plane shear and bending property tests were conducted at room temperature. Displacement and strain fields on the specimen surface were measured by approach of digital image correlation method, and mechanical behaviors and failure modes were analyzed. Results show that the testing program can obtain uniform shear strain field in the test area, so it is suitable for the in-plane shear property tests of mullite fiber reinforced aerogel composites. The in-plane shear modulus and strength obtained by tests are 248 MPa and 0.95 MPa, respectively; the bending modulus and strength are 294 MPa and 2.08 MPa, respectively. Under the in-plane shear load, the cracks initiate near the notch tips and propagate along the path between the two notches. According to the characteristic of bending normal strain distribution, it is found that the neutral plane is not coincide with the geometric symmetry plane. The characteristic of different modulus in tension and compression is validated. The neutral plane position measured by the digital image correlation method is in approximate agreement with the theoretically calculated position, and the relative error is about 10%.