Absolute gravity measurement plays an important role in the field of railway subgrade detection, but most mobile gravimeters have complex structures or mediocre accuracy, and their application range is limited. In this paper, we integrate a system of movable absolute gravity measurement on the rail, which realizes highly integrated system structure design and automatic test process, and undertake experiments of absolute gravity measurement in the laboratory and on the rail. Firstly, in the laboratory, the long?erm gravity measurement sensitivity is 440 μGal?Hz-1/2 and the measurement accuracy is 30 μGal in the integration time of 300 s. Secondly, on the outdoor rail, the uncertainty of movable gravity measurement is less than 15 μGal, and the measurement deviation with the relative gravimeter (LG-1) is less than 40 μGal. Lastly, the gravity measurement sensitivity on the rail is about 707.9 μGal?Hz-1/2. Therefore, our system can realize fast gravity measurement under outdoor conditions and provide new instrument and solution for detection of railway subgrade.