Automatic light adjusting techniques are critical in optical imaging systems. To improve the imaging quality of an optical imaging system, an automatic light adjusting method is put forward. The method is based on the iris with a potentiometer and uses external trigger pulses to set shutter speed. Compared with the light adjusting method which sets shutter speed by using stop alone or by internal synchronization, the closed-loop control of iris and the continuous adjustment of shutter can be realized by using this method. For the hardware, because the DSP in TMS320C28X series is used as the main processing chip, its clock cycle is up to 6.67 ns. With the built-in flash and rich peripherals, the driving pulse for iris motor, the AD conversion of potentiometer, the compilation and the C language programming can be realized. For the software, the light adjusting cycle of 20 ms is used and the flow chart of automatic light adjusting is given. It is verified that this automatic light adjusting method has the features of large dynamic light adjusting range, high reliability, fast response and wide applicability. It can meet the needs of automatic light adjusting in the project.