The invention relates to a data fusion method and a data fusion device for a low-cost integrated navigation system. The data fusion method comprises the main steps of: obtaining a course psiI of a carrier by carrying out strapdown inertial navigation calculation on output data of a microinertia measuring unit in the integrated navigation system; obtaining a course psim of the carrier by carrying out calculation on output data of a magnetoresistive sensor in the integrated navigation system; obtaining a course psiG of the carrier by carrying out calculation on output data of a satellite receiver in the integrated navigation system; and carrying out superposition on the psiI, the psim and the psiG by certain weight according to the set optimum estimated performance index of the course of the carrier, and obtaining an optimum estimation value of the course of the carrier. The embodiment of the invention has the advantages that under the confinement of the optimum performance index with minimum estimated variance, an estimation coefficient can be adjusted automatically according to the dynamic characteristic of the carrier, the optimum course estimation in the moving process of the carrier is given, the self adaptability is stronger, the robustness is good, and the practical application in engineering is convenient.