The invention relates to a construction method of a reinforced concrete box girder, which comprises the following construction steps: 1, placing a steel box girder body on the ground, and pouring andmaintaining bridge deck concrete and flange plate concrete on the ground; 2, installing scaffold units on the ground on the same outer side faces of two bridge piers, and the scaffold units extend into the bottoms of flange plates of a first reinforced concrete box girder; 3, hoisting the first reinforced concrete box girder, loosening hooks, stressing the scaffold units, supporting the flange plates, and therefore preventing the reinforced concrete box girder from overturning due to the eccentric effect; and 4, hoisting the second reinforced concrete box girder, wherein the second reinforcedconcrete box girder is not loosened after being in place, and using a long-strip angle steel to weld with two steel box girder bodies, so that the two steel box girders are pulled and connected into awhole, and the second reinforced concrete box girder is prevented from overturning. The construction method has the technical effects that the construction period of the reinforced concrete box girder is short, and hoisting is safe.