  We systematically investigate the dependence of the temperature of cold
cesium atoms of polarization gradient cooling (PGC) in optical molasses on
experimental parameters, which contain changing modes of cooling laser, PGC
interaction time, cooling laser frequency and its intensity. The SR mode of
cooling laser, that means the cooling laser frequency is changed with step mode
and cooling laser intensity is changed with ramp mode, is found to be the best
for PGC comparing with other SS, RS, and RR modes. We introduce a statistical
explanation and an exponential decay function to explain the variation of cold
atomic temperature on time. The heating effect is observed when the cooling
laser intensity is lower than the saturation intensity of cold atoms. After
optimization, the lowest temperature of cold cesium atoms is observed to be
about 4uK with the number of 2x10^9, a density of 1x10^11/cm^3 and the phase
space density of 4.4x10^(-5). The optimization process and analysis of
controllable experimental parameters are also meaningful for other cold atomic
systems.
