The key technology for map-based variable rate application of fertilizer lies in fertilizer should be applied precisely at each location within the field according to prescription map. Software for variable rate application control was developed based on object-oriented programming approach. A kind of algorithm for real-time application map identifying relying on machine location was designed and successfully implemented. Field test showed that prescription map cell was identified less than 100 ms, which could meet the requirements of actual field application. A model to determine positional lag was developed for map-based variable rate application system by analysis of factors causing position lag. In addition, field tests were conducted and result showed that delay time was about 1.84 s.