The present invention discloses a magnetic field analysis method of a gas insulation wire in a tunnel. The method comprises the steps of: (1) dividing each phase and the housing of a gas insulation wire system into splitting equivalent replacement wires; (2) performing constrained division of the gas insulation wire system according to a transmission circuit sending terminal; (3) calculating vertical self-impedance and mutual impedance; (4) setting a boundary constrained condition of the sending terminal; and (5) establishing a power source external circuit model, and performing prediction andanalysis of the current density distribution and a stray field in the gas insulation wire system. The magnetic field analysis method performs analysis and prediction of the current density distribution and a proximity effect of a multi-conductor system, performs prediction of the power density distribution by employing the Joule loss, employs an integration method to perform assessment of the external flux density, and is simple in method, concise in matrix algebra method and close to the reality in computing process.