The formula of temporal coherence of dynamic subjective speckle was deduced, by which the influence of optical parameters from the imaging system on temporal coherence of speckle was analyzed. It is concluded that the influence of the parameters of screen on speckle temporal coherence can be neglected and the speckle temporal coherence is mainly affected by the diameter of the lens or the size of laser spot on screen. If the size of laser spot is lager than that of resolution spot on object plane, the diameter of lens gives the mainly affection, while on the other side the size of laser spot gives the dominating affection and the temporal coherence can be reduced by reducing the size of laser spot on screen. The result was confirmed by simulation and experiment, and it can give an instruction to speckle reduction in laser display.