To　investigate　the　inversion　of　the　radiometric　brightness　temperatures　of　the　armoured　targets　in　passive　detection　techniques　of　the　millimeter　wave　radiometer,　the　spline　space　and　the　spline　interpolation　method　are　presented　and　applied　to　retrieve　armored　target＇s　brightness　temperatures　from　the　data　of　antenna　temperatures.　The　solution　of　an　ill-posed　Fredholm　integral　equation　of　the　first　kind　which　is　the　mathematic　pattern　of　millimeter　wave　radiometer＇s　antenna　temperatures　is　obtained　and　the　stability　of　the　solution　is　dominated.　The　iterative　formula　for　the　case　of　variable　tendency　and　noiseless　are　also　extended.　The　results　are　valid　in　detecting　and　distinguishing　the　targets.