In order to resolve the problems encountered in the process of forecasting the quantity of equipments demanded by aircraft’s support system during the design and development phase, including the difficulty to collect the input data of the existing forecast models and their imprecise and unauthentic output results, a model of was established to forecast the number of support equipments based on support activity flow described by functional flow diagrams. Firstly, the aircraft’s typical mission was analyzed and the quantity of operation and maintenance support activities happened in the duration of the mission was calculated on the basis of the typical mission’s data and aircraft’s reliability data. Secondly, the working hours and maximum number of the support equipments demanded by the support activities in the bottom of the functional flow diagrams was calculated and analyzed. Thirdly, an approach to sum the working hours and maximum quantity of the support equipments demanded by the support activities with logic relations in the same level of the functional flow diagrams was presented and with which the working hours and maximum number of support equipments demanded by the top-level support activity was achieved. Fourthly, the quantity of support equipments to be allocated in the support sites was calculated. At last, the proposed model was demonstrated via a practical case of aircraft, which proved that the model is corrective and effective.