Abstract: Spectrum information is the best single to respect the droplet transfer discovered by far,but the system based on spectrograph can not apply to test welding droplet on line.Guided by the thought of spectrum sensing droplet transfer,a small type of spectrum sensor of droplet transfer is developed in this paper.The idea of the design and theory explore on the practice system is introduced in detail,also the practice structure of the sensor is give out.The sensor worked well on line control.