The invention discloses an attaching type high-altitude assembling support and a construction method of a rigid frame-continuous girder side span cast-in-place section. The support comprises a high-altitude support body located on the inner side of a pier and a plurality of attaching type connecting structures arranged from top to bottom and is characterized in that the high-altitude support bodyis arranged along the transverse direction of a bridge and fixedly fastened to the pier through the attaching type connecting structures; the vertical high-altitude support body comprises two symmetrical derricks, each attaching type connecting structure comprises a connecting crossbeam and four horizontal connecting rods, and each connecting crossbeam is horizontally arranged along the transversedirection of the bridge and fixedly connected to the inner lateral wall of a pier body through a plurality of pull rods. The method includes the steps of firstly, setting up the attaching type high-altitude assembling support; secondly, erecting formwork; thirdly, casting the side span cast-in-place section. The attaching type high-altitude assembling support has the advantages that the support is reasonable in structure, simple in construction and good in use effect, the derrick type assembling steel-tube support is combined with the traceless pre-buried attaching type connecting structures,the construction difficulty of the high-altitude cast-in-place section can be solved effectively, and a safe and reliable construction process can be achieved.