Abstract: The dynamic penetration test (DPT) is one of widely applied in situ test techniques in bed covering of western region in China. The rod length during operation can be as long as hundreds of meters, and it should be modified in practical applications. On the modification of dynamic penetration rod length, the modified coefficient is proposed only in the range of 20 m in China's standard “Code for investigation of geotechnical engineering”,but how to modify has no provisions when the depth is more than 20 m. So the standard is unable to meet the production needs, and becomes a major issue that limits its applications. Indoor model tests are carried out for the modification issue, and heavy dynamic penetration model are conducted on sand and gravel soil with series of rod lengths (2.0 m, 8.9 m, 16.4 m, 23.4 m, 30.0 m, 36.0 m, 62.0 m, 83.0 m). The modification coefficient of heavy dynamic penetration rod length is obtained after the tests. It is analyzed and concluded that the modification of heavy dynamic penetration rod length obeys the description of Newton elastic collision theory, and is independent of the material properties and overburden pressure. Finally, the modified coefficient of dynamic penetration rod length is proposed.