Abstract: Railway ballast tamping operation is an important work in the railway line maintenance and renewal operation, which is employed to restore the initial geometry of railway track distorted by train traffics. The main goal of tamping operation is to compact the stone ballasts under the sleepers supporting the railway squeezing and vibrations. The ballast compactness is the most direct index for evaluating the quality of tamping operation. This paper presents an experimental method used to measure the ballast compactness under the sleeper before and after tamping operation based on water-filling method and creates a discrete element analysis model of railway ballast which analyzes the change of ballast compactness under tamping operation based on discrete element method. The simulation results are very similar with experimental results, which verify that the discrete element method is an effective method to evaluate the change of railway ballast compactness under tamping operation.