Abstract: The vacuum thermo-cycling tests (93~413 K, 10^-5Pa) were carried out on the unidirectional M40J/5228A composite. The mass loss and linear expansion coefficient of the specimens were measured after vacuum thermo-cycling of different cycles. The effect of vacuum thermal cycles on the linear expansion coefficient profile was analyzed using a proposed two-dimensional model of micro-damage. The results show that the mass loss rate increases with increasing the vacuum thermal cycles, and becomes flat after 48 cycles; the transverse expansion coefficient linearly increases with increasing temperature, and is not affected by vacuum thermal cycles. The longitudinal expansion coefficient linearly decreases with increasing temperature under the original state, while it exhibits a non-linear characteristic of decreasing firstly and then increasing with temperature after 113 cycles. The effect of vacuum thermal cycles on the longitudinal linear expansion coefficient closely depends on the breaking extent of the binding interface and the relief of residual stress.