The　dynamic　characteristics　of　negative　stiffness　Duffing　system　with　delayed　feedback　control　are　investigated.　Choosing　time　delay　as　the　control　parameter,　the　stability　of　trivial　equilibrium　is　discussed　by　analyzing　distribution　of　the　roots　of　the　associated　characteristic　equation.　It　is　found　that　Hopf　bifurcation　occurs　from　trivial　equilibrium　when　the　delay　passes　through　critical　values,　then　the　critical　values　and　their　relations　with　system　control　parameters　are　obtained.　The　effect　of　time　delay　on　the　forced　vibration　of　system　is　evaluated　with　numerical　method.　The　results　show　that　the　amplitude　of　stable　period　motion　increases,　some　complex　dynamical　behaviors　such　as　quasi-periodic　motion　and　multiple　periodic　motion　may　occur,　and　the　system　gets　out　of　control　with　the　increase　of　time　delay.　Therefore　the　effect　of　time　delay　ought　to　be　taken　into　sufficient　consideration　in　the　design　of　system　control　loop.