We have studied the two-photon resonant excitation and enhancement effect of a high excitation state 15d in Rb atom by using a synchronously pumped tunable mode-locked dye laser and obtained the frequency up-conversion at 1.065μm→4210 A. The relationship between self-phase matching and output of UV coherent radiation output is given. This paper emphasizes the study of the transient coherent effect and dephasing process of this state. It has been shown experimentally that the maximun polarized time by two-photon resonant excitation is 120 ps and UV coherent radiation could be induced tinder the action of probing signal until 350 ps. However up to now no report has been declared about the study of trasient coherent dephasing in higher excitation states oi atom systems.