Abstract: This paper mainly explains that after the AC311 A helicopter is equipped with agricultural and forestry spraying equipment, the strength of its connecting structure and equipment under the load conditions of helicopter normal flight, maneuvering flight and emergency landing meet the requirements of airworthiness regulations. According to the applicable provisions of CCAR-27 Part C "strength requirements" and the compliance verification method recommended by AC27 MG5, the strength of AC311 A helicopter equipped with imported agricultural and forestry spraying equipment is verified to meet relevant requirements by means of calculation and analysis. The results of calculation and analysis provide support for the STC certification of AC311 A helicopter equipped with imported agricultural and forestry spraying equipment. This method has certain guiding significance for the strength compliance verification of civil helicopter modification design.