Abstract: For the problem existing in HDD (Horizontal Directional Drilling) crossing construction, the backtow force of pipeline is often considered, but the backtow force of drill rod and bore reamer is neglected. In the paper, the factors to be considered in geological condition of crossing formations, pipeline pulling force, bore wall friction and fluid resistance are proposed, and calculation model for backtow force is established. The backtow force calculation method based on ASTM (American Society of Testing and Material) is introduced and the calculation method considering the backtow force of drill rod and bore reamer are derived. Researchers have performed simulation calculation for two project examples with different stratigraphic conditions, the maximum error between the backtow force considering the drill rod and bore and the actual pulling backtow forece is 2.4%, the calculated results are more closer to the actual backtow force than that of only considering pipeline backtow force.