Abstract: Spacecrafts in low earth orbit are subjected to frequent thermal shocks due to repeated entry and exit of the Earth's shadow area.Therefore, it is very important to study the impact of thermal shock on the mechanical properties and mass loss rate of domestic high modulus carbon fiber/cyanate ester composites used in low orbit spacecrafts.In this paper, the thermal shock tests of CCM40J/cyanate ester composites have carried out, the mechanical properties and mass loss of CCM40J/cyanate ester composites after different thermal shock times have tested, and the effects of thermal shocks on CCM40J/cyanate ester composites have analyzed by SEM, FTIR and XPS.The results show that the tensile strength, tensile modulus, flexural strength, flexural modulus and interlaminar shear strength of CCM40J/cyanate ester composites decrease by 0.90%, 2.13%, 3.72% and 6.49% respectively after 500 thermal shocks.At the same time, the mass loss ratio of CCM40J/cyanate ester composites is far less than 1%, and no new chemical reaction take place.