The plantation in metropolitan suburb is functional in production, ecological service and social guarantee. It is difficult to determine rational plantation preservation in metropolitan suburb for the existing policy based on the rapid urbanization and industrialization. This paper makes a case study of Daxing district in Beijing where is developed in economy, establishes three prognostication models for plantation preservation quantity based on reservation method, provision needs method and regressive analysis, which are employed to prognosticate the plantation preservation quantity based on some supposals, and presents some approaches to plantation preservation quantity.