Provided is an automatic secondary degassing fixed-length mechanism for an ultrathin heat pipe. The automatic secondary degassing fixed-length mechanism comprises an automatic lifting device A installed on a length adjustment sliding table, an automatic clamping device B, a length positioning and extension device C and a PLC. The present invention, having the advantages of simple structure, high efficiency and stability, is suitable for the secondary degassing fixed-length processing of heat pipes of different lengths, and particularly suitable for processing ultrathin heat pipes made of a thin-walled heat pipe by a flattening process, having advanced structural design and stable and high-efficiency production. In this mechanism, size positioning and automatic clamping in the secondary degassing fixed-length process for the heat pipes are correspondingly achieved through the automatic lifting device A and the automatic clamping device B. Downward component force applied to the thin-walled heat pipes in the die-opening-sealing process is released through the length positioning and extension device C, so that deformation of bending or partial sinking of pipe bodies of the thin-walled heat pipes in the secondary degassing fixed-length process is avoided. In this way, the qualification rate of the products and the economic benefit of the enterprise are greatly improved, and the problems with the existing secondary degassing fixed-length processing of the ultrathin heat pipe are solved.