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　most　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.