The outer casing of heater made of iron-boron alloy is brittle fractured easily. It is a an effective way to add continuous fibers in brittleness material to improve the mechanical properties of composites. Temperature and thermal stress distribution of iron-boron alloy casing and the influence of thickness on the thermal stress of the casing is computed with finite element method, and the thermal stress distribution and influence of fiber volume fraction of continuous fiber-reinforced composites is studied. The result indicates that the external surface is in tensile stressed condition, and the internal surface is in compression stressed condition; the thermal stress increases with the thickness. The compression stress of the matrix decreases and the tensile stress increases with the increasing of the fiber volume fraction.