φ42mm rod is regulated for DPH in most standards of China, but a lot of engineering investigation units tend to use φ50mm rods based on the consideration of construction convenience factors. By the analysis on the testing points of DPH and the influencing factors, based on the revision coefficient table of DPH, the derived coefficient of rod diameter conversion is 0.89, which has been verified by a waterway engineering instance in island reefs of Hainan. The results show that for the coral clast formation of buried depth less than 12m in the same geomorphic units, the ratio of hammering numbers of DPH with φ42 drill pipe and DPH with φ50 drill pipe is 0.875, which is in close agreement with the derived results.