To support the requirement of decimeter-level BDS SBAS service, a type of differential correction, the "zone correction", it is developed. It represents the remaining un-modeled errors of pseudo-range and carrier phase observations, and could be encoded into the current broadcast messages. The zone correction is derived using observations of reference stations, where their coordinates are ... [Show full abstract] precisely known. Users receiving the broadcast zone corrections covering its region, use them to correct the observations, thus the observation modeling errors are reduced and positioning accuracy can be improved. It is analyzed that the impact of parameter updating rates and the distance between users and the zone center on the user positioning accuracy. Data of the BDS tracking network in the China continent is used to validate the proposed model. Results show that mean positioning accuracy reaches 15 cm and 20 cm for horizontal and height components, respectively.