Measurement accuracy of γ-ray backscatter ash analyzer is greatly affected by the changes in the composition of the coal ash. In current ash analyzer, calibration is underway anew when measurement error is comparatively large, with a narrow measurement range, low accuracy and complicated operation. In this paper, the impact of changes in the composition of the ash on the accuracy of ash analyzer was analyzed from the working principle of γ-ray backscatter ash analyzer. A calibration model based on the dual low energy γ ray measurement was established，which can improve the measurement accuracy and can be achieved automatically on software. The calibration model was then used on a γ-ray backscatter ash analyzer, and the test results show that the existing deficiencies are overcome with an measurement error of less than 1%，meeting the required measurement accuracy of EJ/T1078-1998 γ- ray coal ash analyzer.