The invention discloses a pre-weld demagnetization method for narrow gap pulse fusion welding of a petroleum drill collar, and aims at solving the technical problem of poor practicability of an existing narrow gap welding groove demagnetization method. According to the technical scheme, a magnetic shielding ring sleeves the outside of a gun body of an existing narrow gap pulse melting arc gas-shielded automatic welding gun; the magnetic shielding ring provides unobstructed flux branch circuits for magnetic lines in grooves; and one considerable part of magnetic lines distributed in the grooves enter a drill collar weldment through the magnetic shielding ring, so that the residual magnetic field intensity of an arc space in each narrow gap groove of the drill collar weldment is obviously reduced. When the residual magnetic field intensity in each groove is measured below 4mT before being welded, the effect of arc magnetic blowing on the quality of bottom-layer welding lines of the drill collar weldment is eliminated; the arc state is stable; and the weld bead formability is good; and when the residual magnetic field intensity is measured over 4mT before being welded, the non-weldable condition is judged and loss of working time and material waste are avoided. An experiment proves that the magnetic shielding effect of the method on the grooves is obvious, the weldable rate is effectively improved, and the practicability is high.