The temperature kinetics of InGaAsP/InP high-power laser diode arrays with different fill factors was studied experimentally and theoretically. It was shown that except during a short initial period, the laser array heating is determined by the heat flux propagation through the heat spreader. To characterize this heating, we developed a two-dimensional analytical model. Experimentally, the temperature change in the active region was obtained by measuring the laser spectrum’s temporal evolution during a single current pulse. Three distinctive periods in the transient heating process were clearly identified—an initial temperature rise, a square-root-of-time dependence of the active-region temperature increase, and an exponential approach of the active-region temperature to its steady-state value. We demonstrated that in the initial period of time, the heat propagates within the laser bar structure, and the laser bar design (fill factor) strongly affects the active-region temperature rise. In the later periods the temperature kinetics is insensitive to the fill factor.