Abstract: Joint coating technology of electro welding sleeve has been widely used in field joint coating for thermal insulation tube of hard PU foam plastic of high-density polyethylene outer sheath. Tensional and shear strength as well as gas sealing are key factors influencing the joint coating quality. In order to ensure high quality, taking the ϕ 365 thermal insulation tube of high-density polyethylene outer sheath as example, the influences of welding parameters and cooling velocity on tensional and shear strength and gas sealing of the joint coating of electro welding sleeve have been studied. The results indicate that, only when proper welding parameters are selected, can the best tensional and shear strength at the place between electro welding sleeve and major tube as well as axial bridging part of electro welding sleeve be achieved. Moreover, fast cooling will lead to the decrease of tensional and shear strength of the electro welding part. Therefore, measures of thermal insulation shall be taken under the cold environment. The gas sealing of joint coating is not only affected by welding parameters, but also subject to the field operation to a great extent. Combined with the experimental results, the key points for the operation of joint coating of electro welding sleeve are put forward.