The defects in the inner wall of three dimensional hollow cylinder-shaped equipment are calculated by means of infrared technique and solution of inverse heat conduction, and correctness of the solution is proven. Inverse solution to find the geometric shape with partially unknown system will lead to the solution of one nonlinear algebraic equation under the condition of the given temperature of the surface. The Newton iterative method can be used to resolve this equation for determining the position and size of the defects accuratly.