Abstract: The on-line analysis instrument of iron ore slurry grade is researched by using the absorb method of double energy γ ray. This method can be used for the on-line measurement and the control of the iron ore slurry grade in the process to select the iron ore. The system has good adaptability to the variety of spot's technical condition. The variety of concentration and grain of ore slurry affect measurement of grade very little. The error of grade measurement is less than 0. 5% and the long stability is less than 0. 05 %