In order to improve the gray-scale characteristics when the traditional analog driving scheme is dimmed, a PWM circuit for silicon-based OLED driving chip has been presented. The circuit could modulate the common cathode voltage applied to the OLED array with PWM, so as to achieve the same gray scale characteristics when adjusting brightness. The circuit was designed and verified by the 0.18 μm mix-signal 1P6M CMOS process. Simulation and test results show that when the brightness is adjusted from 100 cd/m² to 500 cd/m², the gray-scale characteristics could be significantly improved.