Based on the analysis for the course of the photochemical reaction in MBDCG during exposure, the dredging-blocking model for strengthening the sensitivity of MBDCG is advanced, and the viewpoint that blocking is more potencial than dredging is suggested as well herein. The double-sensitive-centre method is presented and its principle for stenthening sensitivity is discussed as an example for the dredging model. At the end, the practical sensitizing effect by this method is investigated as well.