Radius　basis　function(RBF)　is　a　kernel　used　in　SVM　widely.　Tuning　kernel　parameters　has　a　significant　impact　on　the　performance　of　SVM　generalization,　K-fold　Cross-Validation　is　usually　adopted　in　the　selection　of　kernel　parameters　at　present,　but　it　has　high　demand　on　computation.　Analyzed　the　meaning　of　Gram　matrix　constructed　by　kernel　function,　an　ideal　Gram　matrix　for　demonstrating　the　similarity　between　samples　is　defined;　To　select　the　parameter　of　RBF,　a　novel　method　which　measures　the　similarity　between　the　ideal　Gram　matrix　and　the　Gram　matrix　obtained　through　RBF　kernel　by　distance　measurement　is　also　presented.　Theory　analysis　and　experiment　results　show　that　the　presented　approach　not　only　has　a　good　precision,　but　simplifies　calculation　evidently.