An electro-optically fast-tuning ruby laser, which consists of a calcite-KDP crystal as the fast-tuning component and a Fabry-Perot resonant frequency selector as the fine wavelength controller, has been developed for a differential absorption lidar(DIAL)system for water vapor sounding in the atmosphere. The laser has stepwise tuning characteristics and emits two pulses in a single shot, with the first pulse tuned at the center of an absorption line of the water vapor and the second one in the nearby valley. Operation theory and tuning characteristics of the laser are discussed in detail.