Raindrop spectrum observation is a key to acquiring the raindrop characteristics. To build small watershed model is an effective way to do experiment in all kinds of small drainage areas. Based on the similitude ratio and the small raindrop of model rainfall and the difficulty of measuring raindrop spectrum characteristics, the authors tried to use traditional Color-spot method with CorelDRAW to form a new method for data processing and obtained the raindrop spectrum characteristics. It proves that the new method can improve measuring precision and reduce the workload greatly.