The evolution of light pulses from a synchronously pumped transient mode-looked rhodamine dye laser has been caloulated by using the rate equation method. The transient characteristics of the system with only 12-round-trip modulation are analysed. The real time observed on a streak camera with 1.25 (ps/ channel) is in a qualitative agreement with the computed results. Effects of the pump power on the laser pulse width are calculated. We found that the laser pulses first become shorter, then become longer and finally tend to a dynamio equilibrim. The gain. modtuation plays a role only in "the former part of laser poise round-trips； the laser pulse "width becomes longer as the pump increases.