The present invention discloses an extraction magnetic channel first harmonic shimming device in a superconductive circular accelerator. The device comprises an electrostatic deflection plate, an extraction magnetic channel and a compensation element which are arranged at the inner wall of a thermostatic chamber of a superconductive circular accelerator along an extraction path. The compensation element is arranged at a proper position at the inner wall of the thermostatic chamber of the superconductive circular accelerator and configured to offset the influence of a harmonic field caused by the extraction magnetic channel on a main magnetic field. The present invention further discloses a method: compensation iron blocks with different radiuses are moved at different angles are moved to perform measurement of the magnetic field to obtain the optimal shimming position. Through change of the shape and the position of the compensation element, an unexpected technical effect is generated,the compensation iron blocks with proper sizes are arranged at the proper positions of the extraction path, the generated offset magnetic field is nearly identical to the curve of a magnetic channelMC2-6 harmonic field, the compensation effect approximately reaches 100%, and therefore, the compensation effect of a harmonic coil is more obvious compared to the prior art.