The stability of semiconductor lasers depends on the driven power. According to the requirement for light extinction ratio tester, a practical semiconductor laser driven power based on the UC3842 PWM chip was designed. The driven power is applicable to the smaller power semiconductor lasers, the output duty cycle and frequency adjustable driven signal, which make it put out some modulation optical signals with a certain frequency, so the protection functions, such as slow start, over-current, over-voltage and high frequency can be realized. These protections can make semiconductor lasers working safely at room temperature. The experimental results prove the feasibility of this program and meet the requirements for tester.