A closed fixture device for replacing a single insulator and a replacement method thereof are provided, the device includes a first fixture, a second fixture, tightening assemblies and extension joints, where the first fixture and the second fixture are respectively clamped and positioned on an insulator string; a retractable bearing structure is formed by connecting two the extension joints and the tightening assemblies between the first fixture and the second fixture; and the first fixture and/or the second fixture include a main clamp and an auxiliary clamp cover and are connected by pin shafts, special-shaped articulated bolts and nuts; a center of the main clamp is a semi-circular arc body, with symmetrically arranged ear plates on both sides; the ear plates are provided with connecting holes for connecting the extension joints or the tightening assemblies; the special-shaped articulated bolts are symmetrically assembled on the main clamp.