The utility model relates to a water storage device of the technical field of the filtration of the water purification, particularly a no-pressure water storage tank for a small-sized water purification machine, which is composed of a water tank casing body, an air delivering pipe, an air storage air bag, a magnetic ball floating switch, a water discharging tap, a water inlet pipe, and a cover casing. The utility model is mainly characterized in that after the pure water entering into the water tank, the air in the water tank enters into the air storage air bag outside the water tank casing body through the air delivering pipe. The effect of negative pressure can not be generated at the water discharging time, so that the water can be discharged. Using the water tank, the utility model can avoid the microbial secondary pollution of the water purification machine caused by using the open water tank, so as to reach the purpose of sanitary pure water.