Research on the gain on ignition question for preparing ore with fusion sample preparation technique was done, and a new correction method was worked out. After fusion sample preparation, sulphide ore would have a lot of sulfate, which would change the sample matrix dramatically, and the matrix effect was corrected with errors. In this paper, a new solution to measuring gain on ignition was put forward, and it was committed step that the sample having gain on ignition was redefined by sample having loss on ignition. Through conversion, the gain on ignition was corrected by subtraction method, and the matrix effect was corrected by theoretical influence coefficients. The analysis results of redefinition samples were converted into the primal samples lastly. The project took copper concentrates as the experimental object, it was found that the gain on ignition of this method was obviously different from the gravimetric method, but through influence evaluation, this error had little effect on test results, and may be reduced by flux. After correction of the gain on ignition, the standard deviation of the elements calibration curves for Cu, Fe and S were less than 0.14%, 0.074% and 0.14% respectively, which were obviousely superior to 0.21%, 0.15% and 0.19%. The results got from this method were in satisfactory agreement with certified values.