Spontaneous drop of slag on wall tube at the time of boiler load shift was studied theoretically. Slag thermal stress was assumed to be composed of slag shaping thermal stress and temperature gradient thermal stress.A model was established to calculate slag thermal stress. The distribution and quantity of slag thermal stress created by boiler load shift were calculated and analyzed. Thermal stress could be as great as E/1000 and be enough to destroy slag structure and to make some fall off.Wall tube parameters could only affect thermal stress of the slag which is 1mm near the contact surface and contribute little to slag fall-off.