The invention provides a method and a system for grading and incinerating wastes containing biomasses in a coal-fired boiler. According to the method and the system, a decomposition incineration platform is arranged into a plurality of hollow pipelines to be arranged in parallel on one hand, a certain gap is reserved among the hollow pipelines, the pipelines are used for supporting a retention space or remaining space of a waste containing the biomasses, the advantage of high-temperature incineration of the coal-fired boiler is utilized, so that the waste containing the biomasses fed from a first feeding port can be fully combusted, and on the other hand, a furnace bottom incinerator grate composed of a rotatable roller belt with a strip-shaped surface is ingeniously combined with the pulverized coal boiler of a thermal power plant, the high-temperature waste heat of a cold ash bucket area under the hearth of the pulverized coal boiler of the thermal power plant is utilized, the wastecontaining the biomass is fed from a second feeding port to be incinerated to remove harmful pollutants, so that the whole waste treatment efficiency is high, the effect is good, the investment of a special construction waste incinerator is reduced, and the operation cost is greatly reduced; the waste of the biomass can be removed by utilizing the coal-fired boiler, so that the recycling of the garbage is realized; and meanwhile, the high-temperature airflow of the coal-fired boiler is used for fully decomposing dioxin, so that the harmless waste of the waste is realized.