The invention discloses a river crossing bridge reconstruction and extension construction method. The method comprises the following steps that an cofferdam is built at the preset position in a riverchannel; a concrete foundation is poured in the cofferdam, a temporary support is arranged on the top surface of the concrete foundation, and a stiffening plate is arranged on the upper top surface ofthe temporary support; a jack is arranged at the preset position of the top of the stiffening plate, and a steel box beam is arranged above the jack; beam covering concrete is poured on the upper portion of the steel box beam; the cofferdam is removed, and the step 2 is executed after the cofferdam is built at the next preset position; when the beam covering concrete strength reaches a design allowable value, the temporary support is unloaded and removed. According to the river crossing bridge reconstruction and extension construction method, by means of the steps, the construction period ofriver crossing bridge reconstruction and extension can be effectively shortened; meanwhile, construction is conducted through the method, the construction period limitation of river crossing bridge reconstruction and extension engineering can be greatly extended, when the flood season is around the corner, the river crossing bridge reconstruction and extension becomes possible, and the construction cost is effectively lowered.