The invention relates to an overall installation method of a double-body separable combined beam, a temporary connection system (200) is arranged between a left separable steel box beam and a right separable steel box beam at the rear end of a section of the combined beam (100) to be installed, and matched connectors (300) are arranged in the positions of webs of the left separable steel box beam and the right separable steel box beam, so that the cross sections of a beam section to be erected and an erected beam section are connected in a matched and abutting mode. In the installation process of the section of the combined beam (100) to be installed, a beam erection lifter (400) lifts the combined beam (100) to be installed to slowly approach the front end of an installed combined beam (100), alignment of a section to be erected and an erected section is carried out through the matched connectors (300) and bolts, and accurate positioning of end surfaces of the beam is achieved. The overall installation method of the double-body separable combined beam reduces the relative deformation of flanges at two sides of the double-body separable combined beam under a lifting state, and resolves the problem of the aligning of the cross sections of the beam section to be erected and the erected beam section.