Aiming at the problems of low efficiency, high labor intensity and difficult to reduce the measurement uncertainty in manual measurement of metal structure density,by analyzing the existing manual measurement scheme, the key factors affecting the measurement quality were obtained; combined with the key factors affecting the quality, the overall function design was carried out, and the overall structure, measurement process and control principle of the automatic measurement device were obtained; the uncertainty of the measurement results was evaluated, and the calibration standard of the density test results was obtained to ensure the accuracy of the test data; the effectiveness of the device was verified through metal structure measurement experiments. The results show that compared with the manual measurement method, the measurement uncertainty of the automatic measurement device is reduced by 75%, and the detection efficiency is improved by 180%.