A weighted area scheme is designed for calculating mean value of a regional meteorological field. The method is used to calculate the mean annual precipitation, the water vapor content of January and July over northwestern China. Compared with arithmetic method, Tyson polygon method as well as grid point interpolation method, the method is regilable to calculate the aerography field hypodispersion in macroscopy area, if there have more sites and these sites are nonuniform distribution. Its advantage is smaller computational complexity and more slight influence of the lacking observation.