Abstract: Observing the solar magnetic field from a single point, we can measure the magnetic field along the line of sight and the magnetic field perpendicular to the line of sight (transverse field). However, the accuracy of transverse field is one order of magnitude lower than that of the radial field, and there is a 180^\circ uncertainty in the direction of the transverse magnetic field. The goal of this paper is to improve the accuracy of the transverse field of the solar magnetic field. It is considered to observe the magnetic field at L5 point and Earth simultaneously, and the error reduction can be obtained through model construction and data analysis. The specific methods are as follows: Coordinate relations of magnetic fields at Earth and L5 point are obtained by spherical trigonometry formula, and the corrected errors of the transverse magnetic field at L1 points are obtained by error transfer formulae; Calculate the error distribution of the solar magnetic field data, and simulate two data conforming to the solar magnetic field distribution,which are respectively used as the error diagram of Earth and L5 point; By combining the data of Earthand L5 point, the corrected data of the transverse field error at Earth are obtained and compared with the original transverse field data. It is found that the transverse field error of Earth can be reduced to about 17% of the original.