Abstract: Abstract: Solar water heaters are widely used in urban and rural China. The Home Appliances to the Countryside program from government further expands its application in rural areas. In the end of 2010, there were 168 million square meters of installed solar heaters, with an annual primary energy saving of 20 Mtce. The total installation is very likely to reach 250 million square meters in 2015 and at least 600 million square meters in 2020. In the 2009 Copenhagen UN Climate Change Summit, president Hu Jintao pledged to the international community to reduce the CO2 intensity of the economy by 40% to 45% by 2020 on the baseline level of 2005. Meanwhile, the share of non-fossil energy is also expected to rise to 15%. Application of solar water heaters will help to achieve this goal. The thermal performance of the flat-plate type solar energy water heater has been studied globally. In order to ensure the efficient operation of the solar water heater and user's demand to the terminal temperature of a tank, the countries all over the world with different climate have given different recommended value for water-mass-to-collector-area ratio (the ratio is abbreviated as MAR) of the flat-plate solar water heater. However, China has a vast territory and its climate is complex. The value range of recommended value from literatures is too large to be used to practical application. The main residential buildings in large and medium-sized cities are mostly high-rise buildings. The solar water heater installed on roof can only meet hot water use for the top six to eight floors. The application of the balcony wall-mounted solar water heater is one of the effective ways to solve hot water needs for the rest of users in high-rise buildings. Based on the typical meteorological data of nine cities in mild region of China, the values of MAR of the balcony wall-mounted flat-plate solar water heater have been calculated by using the established mathematical model. The water tank terminal temperature of 60℃, the collector angle of 60°-90° and the azimuth angle of 0°-90° were used in model analysis. Calculation results for south-facing balcony wall-mounted solar water heater in mild region showed that the appropriate MAR of spring, summer, autumn, winter and the whole year was 28-51, 21-41, 31-53, 37-57 and 31-47 kg/m2, respectively. For convenience of practical application, the linear regression relation between seasonal and annual average MAR and the tilt angle for south-facing balcony wall-mounted solar water heater. The correlation coefficients between them were greater than 0.99. In this paper, we introduced a new concept "azimuth factor of MAR" which was easy to calculate MAR for the collector with different azimuth angle. For the non-south-facing balcony wall-mounted solar water heater, the seasonal and annual average azimuth factors of MAR decreased with the increase of the azimuth angle. The seasonal and annual average azimuth factors of MAR ranged from 0.57 to 1.00 and 0.72 to 1.00 when the tilt angle and azimuth angle were respectively at 60°-90° and 10°-90°. The azimuth angle had about 5% effect on MAR for winter and spring, summer, autumn and the whole year when the azimuth angle was less than or equal to 20° and 30°. The azimuth angle had about 10% effect on the above-mentioned MAR when the azimuth angle was less than or equal to 30° and 40°. In conclusion, the annual average solar fraction ranges should be in 0.55-0.70 for the south-facing balcony wall-mounted flat-plate solar water heater used in mild region of China.