Abstract: With the in-situ gravel-rail as trial materials, mean gradation is determined by particle size analysis. On the basis of the mean gradation, test grading is designed with substitution method. According to the character of gravel-rail, the water content of samples is determined according to the content of clay. The laws of the compactibility with water content and gravel content are studled by the samples made under same vertical pressure. The formula of optimum gravel content is deduced from theory. With the analysis of theory and experiment, the methods of determining the optimum gravel content and optimum water content of gravel-rail are presented.