The economic target for the civil aircraft WWS is partly contributed by the system weight load during the flight. To reduce the system weight load and improve the system economy, this document does a research on the water fill requirement according to the flight course accounting data, and then comes to a calculation method of the needed water volume. The comparison between the calculation result and the data of other aircraft water tanks shows that the calculation method is effective. According to the distribution characteristic of the flight hour in our country, the pre-selection water fill function which is based on the flight time and attendants numbers has been studied. The research shows that the the pre-selection water fill function could reduce the flight weight effectively. The study in the document could be used in the civil aircraft water and waste system research and the optimization design.