The invention relates to the technical field of pipe fittings for connection of pipelines for conveying fluid, in particular to a site thermal insulation structure of a prefabricated heat preservationstraight pipe and a special pipe fitting. The site thermal insulation structure comprises the special pipe fitting (1) and the prefabricated heat preservation straight pipe (2) which are connected ina sleeving manner; and a thermal contraction belt for sealing is annularly arranged at the connecting part of the special pipe fitting (1) and the prefabricated heat preservation straight pipe (2). The thermal insulation structure is good in sealing property and the mounting of the thermal insulation structure is free from space limitation. The invention also relates to a construction method of the thermal insulation structure. The construction method comprises the following steps that the special pipe fitting (1) and the prefabricated heat preservation straight pipe (2) are connected in thesleeving manner, and the connecting part of the special pipe fitting (1) and the prefabricated heat preservation straight pipe (2) is annularly wrapped by the thermal contraction belt (3) for sealing.With the adoption of the construction method, the site thermal insulation structure of the prefabricated heat preservation straight pipe and the special pipe fitting is good in environmental protection performance and sealing property, and the foam density of a thermal insulation layer is high.