Both numerically and experimentally, the temporal coherence of parametric fluorescence pumped by picosecond laser is studied. The influence of pump power and pump pulse width on temporal coherence of parametric fluorescence is investigated using the two-time mutual coherence function in the presence of walk-off between the signal and idle. The numerical simulations show that a suitable pump power and a relatively short pump pulse width are very important to increase the temporal coherence of optical parametric fluorescence while a too high pump power or a relatively long pump pulse width can reduce the temporal coherence. Using the periodically poled lithium niobate crystal, the theoretical result is preliminarily proved experimentally.