A high-power laser diode driving controller (HPLDDC), which incorporates the power converter with the feedback controller, was developed and implemented in this paper. The synchronous buck–boost converter (SBBC) was the topology of the power converter; the SBBC could be operated in step-up or step-down mode in accordance with variable on-board battery voltage inputs into the HPLDDC. Moreover, the feedback controllers were equipped with a current-loop controller (CLC) and a voltage-loop controller (VLC); the VLC was employed to regulate the SBBC output voltage to drive and start-up the high-power laser diode (HPLD). The CLC was used to regulate the SBBC output current to supply a constant-current driving the HPLDs. During the start-up transient phase, when the SBBC output mode is changed from the constant-voltage to the constant-current, a start-up current spike occurs that can destroy the semiconductor material of the laser diode. However, few studies have discussed methods of coping with this problem. Therefore, this study proposed a proportional-integral associating proportional (PIAP) control technology, which can be applied to the CLC for the start-up current spike mitigation. Complete designs and analyses are presented in this paper. Simulations and experiments validate that the PIAP control method is effectual to solve the start-up current spike.