Payload calculation and corresponding payload platform development are important steps in the design and development of a UAV. Because an accurate payload calculation can ensure that the structural load bearing and the force transmission of the airframe is reasonable, and achieve the lightweight design goal of the UAV airframe. In this paper, we propose a method for flight load calculation considering inertial loads, which is based on a fluid grid computing unit to study the in-cell flight load algorithm. Then a platform for developing UAV loads is written based on the method. Finally, the method and the development platform are verified to be reasonable and correct by some data, and have been successfully applied in the design process of a certain high-speed aircraft.