The passive detection techniques of the millimeter wave radiometer play an important role in homing guidance of anti-armour missile and end-sensing-cartridge. In order to obtain the radiometric brightness temperatures of the armoured target, a Fredholm integral equation of the first kind must be solved. This equation is ill-posed and its solutions are extremely unstable. The character of the spline interpolation and the condition of the minimum norm of first order differentiator are used, then the smooth interpolation solutions of armored target’s brightness temperatures are obtained, and are used in millimeter wave passive detection.