The invention relates to a gantry crane girder structure construction method. A gantry crane girder structure comprises girder rigid leg sections, middle double-box girder sections and girder flexibleleg sections and is characterized in that the concrete construction method comprises the following steps of S1, a girder overall assembly process; S2, manufacturing and processing of flanges for connecting the girder rigid leg sections and the rigid leg upright columns respectively; S3, structure manufacturing of the girder rigid leg sections; S4, structure manufacturing the girder flexible leg sections; S5, manufacturing of the middle double-box girder sections; S6, closing of a girder; and S7, assembly of the girder rigid leg sections and the girder flexible leg sections. The keys of the construction method are as follows: firstly, the size requirements for a total length, a beam arch, lateral bending, the flatness of a rail bearing top plate and wave deformation of a web plate of the girder are controlled; secondly, the precision requirements for various sizes of the girder are ensured by replacing curves with lines and through various welding deformation control process means; andmeanwhile, by adopting a method of performing matching processing piece by piece and then performing mounting, the production cost is lowered, and the construction efficiency is further improved.