The　neural　network　diagnosis　method　based　on　frequency-domain　kernel　of　nonlinear　circuit　is　presented.　Each　order　frequency-domain　kernel　can　be　obtained　using　Vandermonde　method　from　the　circuit　response　under　all　states;　circuit　fault　features　are　extracted　and　preprocessed,　which　are　combined　with　circuit　work　modes　and　regarded　as　input/output　samples　of　the　neural　network.　Training　sample　and　test　sample　are　used　to　train　and　test　the　neural　network　respectively;　and　circuit　fault　diagnosis　is　realized　using　the　neural　network.　The　recursion　formula　and　its　solution　method　of　each　order　frequency-domain　kernel　are　given;　and　a　nonlinear　circuit　fault　diagnosis　example　based　on　recurrent　neural　network　is　used　to　verify　this　method.