The present invention discloses a method which applies a nuclear radiation field theory for measuring the flyash material level, firstly the boundary condition of the flyash bucket is assured; the interface of flyash in the ash bucket is leaded to approximate to the surface-shaped radiant body, and the relationship between the boundary of the surface-shaped radiant body and the boundary conditionof the ash bucket is established; according to the mathematical model of exposure rate of gamma filed at the upper part of the interface surface-shaped radiant body, the corresponding relationship between the gamma exposure rate and the height of the material level is established; the actually measured gamma exposure rate is taken into the corresponding relationship, then the ash bucket inner material level height corresponding to the gamma exposure rate is obtained. The method of the invention uses the theoretical knowledge of the nuclear radiating field, the flyash interface in the ash bucket is taken as the surface-shaped radiating body for considering, the relationship between the exposure rate of the gamma field at the upper part of the surface-shaped radiating body and the height ofmaterial level in the ash bucket is established, only with the boundary condition of the ash bucket and the measured gamma-field exposure rate the height of the material level in the ash bucket can be obtained. In the practical application the invention has the advantages of practicability, simple operation and accurate data.