Gas cooled microreactors are small modular prismatic high-temperature gas cooled reactors that use helium gas as the main heat transfer fluid. Helium gas is a medium with excellent thermodynamic properties. This article focuses on the design of the main pipeline of a gas cooled microreactor, using a coaxial double-layer pipeline design (with the hot gas duct as the inner tube and the shell as the outer tube) to connect the reactor pressure vessel and helium turbine, achieving core heat export and power generation. By comparing and analyzing the main pipeline designs of high-temperature helium reactors at home and abroad, a double-layer pipeline design structure is proposed, with 750 ℃ high-temperature helium flowing inside the inner pipeline and 450 ℃ low-temperature helium flowing inside the outer pipeline; Detailed analysis and calculation of the structural dimensions of the hot gas duct and shell were conducted, and the inner wall temperature of the hot gas duct under different insulation thicknesses was evaluated. CFD simulation was used to investigate the heat transfer characteristics between the hot gas duct and the pressure sleeve, in order to explore the influence of different insulation thicknesses on the helium heat transfer process inside and outside the hot gas duct. The results indicate that there is no significant change in the temperature of helium gas along the axial direction in the hot gas duct and pressure sleeve. When the thickness of the insulation layer is less than 30 mm, the temperature of helium gas near the tube wall significantly decreases. However, when the thickness of the insulation layer is greater than 50 mm, it can ensure that the core helium gas temperature inside the hot gas duct exceeds 745 °C, which can meet the temperature reduction requirements for the main circuit transmission and better achieve heat transfer in the gas cooled microreactor core.