Parametric fluorescence is the intrinsic quantum noise of optical parametric amplifiers based on optical parametric chirped pulse amplification(OPCPA) technology. It does not have the same temporal chirp characteristics with the main laser, forms an incompressible pulse base in the compressor, and reduces the signal-to-noise ratio of the laser pulse. In the case of small signal gain, the evolution laws of parametric fluorescence pulse width with different pump profiles, gains, system bandwidths and chirp rates are given based on the analytical formula. Results show that the single stage amplifier output fluorescence pulse width is proportional to the pump pulse width, and it decreases with the gain increasing. With a constant gain, the fluorescence pulse width becomes larger with the increase of the super Gaussian wave order. For OPCPA system, the stretch-compression system has the effect of dispersion broadening,so the final output fluorescence pulse width is closely related to the system bandwidth and chirp rate. Based on peta-watt OPCPA (PW-OPCPA) experimental platform, the relevant verification experiments are carried out, and the experimental results are in good agreement with the theoretical results.