The high-speed electro-optical modulating technology of the high-power laser is the key technology for space optical communication transmission system. This system adopts a fiber-optical coupled laser, so it has effectively decreased the spoilage of transferring medium to the laser power. In order to obtain high-power, high-speed laser modulation signal, the system adopts 1mm-diameter of the LiTaO3 electro-optical modulator. The bandwidth of the electro-optic modulator of this system is 1 GHz, and modulation type adopts the outer modulation. Moreover, in order to ensure the modulator work in the linear region and reduce the requirement to electro optical modulator drive voltage, this system uses the automatic offset control technology. It shows that the system can satisfy the requirement of 300 Mb/s laser modulating velocity and 100 mW output power by sinusoidal signal and square signal experiment.