The invention discloses a new and used pavement splicing structure and a construction method for an ultrahigh section in a highway extension project, and aims to solve the problem that an asphalt pavement structure layer is damaged because of impeded drainage of a new and used pavement spliced part of an ultrahigh section in a highway extension project. According to a scheme I, a longitudinal macadam sewer is formed in a spliced part of a new and used asphalt concrete pavement of the ultrahigh section; according to a scheme II, a waterproof sealing layer is arranged between a spliced widening structure layer and a base layer contacting an asphalt concrete surface layer; according to a scheme III, a spliced widening side surface is coated with a layer of hot asphalt. Due to adoption of the waterproof sealing layer, water of the asphalt pavement structure layer of a spliced widening section can be prevented from penetrating into the base layer longitudinally; as a side surface of a used widened part is coated with the hot asphalt, water of the asphalt pavement structure layer of the spliced widening section can be prevented from penetrating into a structure layer transversely; due to adoption of the longitudinal macadam sewer, longitudinal and transverse accumulated water of a spliced asphalt pavement structure layer can be collected. Therefore, the problem of water damage on spliced parts of a new pavement and an old pavement of the ultrahigh section in the highway extension project can be effectively solved.