Abstract: Flight load calculation has great importance to flap design of transport aircraft. Having been used for decades, traditional method is not sufficient for further design and type certification. According to the results of flight test(FT) and wind tunnel test(WT), it has been discovered that the design load of flap is not conservative in cruise and spoiler deflection configuration. For both situations, flap load is distinctly larger than traditional result, which will cause redesign works and certification issues. By analyzing test data, and using CFD tools, key reasons to both problems have finally been found. For cruise configuration, pressure coefficient of flap cabin was often set to zero approximately in previous design, however, in real conditions, the airproof measure between flap and main wing trailing edge often fails in spite of elaborately design and manufacture; moreover, the spoilers installed before the flap may drift and deflect slightly during the flight. These can cause the flap cabin pressure coefficient less than zero and lead to a flight load increment. As to the other problem, the spoiler deflection in high-lift configuration can result in an upwash to the flap. As a result, the flap load, especially for small flap deflection configuration, increases significantly, which may be even greater than maximum flap load with non-deflection spoiler. Improved methods, which have been proved in the latest airliner design program, are given in this paper to correct the traditional flight load calculation. The key point of new methods is to simulate the pressure distribution in flap cabin more accurately for the first problem, and estimate flap load increment caused by spoiler deflection based on maximum normal force of different flap deflections for the other problem. The results, based on new methods, comply with physical rules and real flight conditions more relevantly.