In order to calculate the feasible region of stress of the main beam of the long-span mixed-composite cable-stayed bridge under dead load , the main beam is divided into two parts: concrete beam (including the side span concrete main beam and the mid-span concrete bridge panel) and steel main beam. Combined with the requirements of the bridge code on the limit of tensile and compressive stress of the two different materials of the main beam, the variable action frequency, standard and basic combination are considered. The formulas for calculating the standard value of constant load stress of concrete beams and the design value of constant load stress of steel main beams of long-span mixed-composite cable-stayed Bridges are derived, and the calculation is carried out by taking the main bridge of Beijing-Hong Kong-Macao high-speed Hanjiang River Bridge as an example. The calculation results show that the feasible region of constant load stress of concrete beam and steel main beam characterized by standard value and design value is suitable for the current standard and convenient for calculation. The calculated feasible region of constant load stress can provide the basis and guidance for the determination of the reasonable bridge state and the design of the longitudinal prestressed tendons of the main girder of the long-span mixed-composite girder cable-stayed bridge