Semiconductor laser is widely researched and applied with the characteristics of small size, light weight, low drive power and current, high efficiency, long operation life, being directly modulated, easy to be integrated with optoelectronic devices, being compatible with semiconductor fabrication technologies and mass-produced. Researching and improving laser driving circuits have great significance. Small power tunable semiconductor laser has particularly high requirements on driving current, small changes in the driving current will directly lead to the fluctuations of output light intensity. In order to achieve a stable output power for semiconductor laser, a constant current driving circuit containing slow start and current limit protection circuits is designed based on current negative feedback principle. In the circuit design, simple devices and design are used as possible to improve the normal operation of lasers. In modulation process, low-frequency distortion and high-frequency switching modulation circuits are designed based on the different characteristics of amplifiers and triode respectively. The feedback current is modulated based on the principle of two amplification negative feedback. The bandwidth of closed-loop is lowed and the stability of closed-loop negative feedback is enhanced.