The photo-acoustic (PA) spectrum of nitrogen dioxide (NO2) in the range of 420-520 nm with a Nd:YAG pumped optical parametric generator and amplifier as radiation source is presented. The spectrum has a characteristic of banded structure superimposed on continuum. The banded structure of the spectrum can be assigned to NO2 B2B1<--X2A1 transition. While the continual one comes from the chaos states of the first excited state A2B2 and the high vibration levels of the ground state X2A1. The relationship of PA signal with buffer gas pressure and NO2 concentration is measured. The PA signal intensity increases with buffer gas pressure and almost is invariable when the buffer gas pressure is more than 3.00*10^(4) Pa. The PA signal intensity has linearity with NO2 concentration. The detection limit is about 2*10^(-5) on the basis of SNR = 1, however lower value of the detection limit can be expected by improving the apparatus properties.