Carbon fibers reinforced silicon carbide composites (C/SiC) are the most promising materials for next generation rocket engine components due to their unique properties, such as low densities and extremely high temperature resistance. The intensity and wall thickness design of C/SiC composites combustion chamber is one of the key tasks for rocket engine design. Based on the thin shell theory and the fourth intensity theory, this research deduced an ideal thickness calculation formula for composites chamber, which was used to calculate the wall thickness of a certain type of rocket engine combustion chamber. The validity of the formula was checked through subscale C/SiC tube by burst-test and thrust chamber by hot fire test. The formula and results proposed in this paper can be widely applied to the design of the wall thickness of C/SiC composites combustion chamber.