Resonance enhanced multiphoton ionization (REMPI) spectrum of NO in the region of 490～580 nm is gained by using optical Parametric oscillator and Amplifier (OPO and OPA) as radiation source. The OPO/OPA is pumped by a Nd∶YAG laser. The spectrum is composed of regular band progressions. In comparision of the results of theoretical calculation with the experimental ones, the peaks of the spectrum are attributed to the C2Π←X2Π transitions. The ionization pathway can be shown as NO(X2Π)hνNO(C2Π)2hν (or hν)NO++e. this conclusion is further verified with the near quintic variation of the ionization signal versus laser intensity. The possible reason that the distribution of the spectral intensity does not follow Franck-Condon principle is analyzed and discussed. The molecular constants about NO(C2Π) state are calculated based on the experiment. The results are e=(2354.9±6.4) cm-1, ee=(14.7±2.5) cm-1 and force constant k=(2.44±0.08)×103 N?m-1. The conclusion can provide reference for the detection of pollutant NO.