With the growing emphasis on bio-engineering techniques, the sustainable advantages of using trees as barriers against rockfalls have become increasingly evident. The key mechanism for forest protection against rockfalls is the dissipation of block kinetic energy during impacts. However, previous studies have primarily focused on the overall attributes of protection forests, with limited attention to the quantitative relationship between internal spatial structural parameters and protective effectiveness. This study systematically investigated the effects of tree diameter, plant spacing, and arrangement pattern on rockfall energy dissipation through physical experiments. The results indicate that: (1) The energy dissipation capacity of trees increases with tree diameter; however, the rate of increase declines significantly when the relative diameter (the ratio of tree diameter to block size) exceeds 0.4. (2) Rockfall energy dissipation increases with reduced plant spacing, but the resultant gain exhibits a diminishing trend. (3) Under otherwise identical conditions, the rhombus arrangement pattern achieved a significantly higher rockfall energy dissipation rate (82.67%) than the square pattern (49.28%). Based on the experimental findings, an optimized protection scheme was designed for a typical rockfall on the slope of the Lehong Tunnel in Yunnan Province, southwestern China. Three-dimensional numerical simulation validated the designed scheme. The designed protection forests dissipated 89.49% of the kinetic energy from 0.5 m blocks, demonstrating the practical efficacy of the parameters derived from experiments. This study quantifies the influence of internal spatial structure parameters on the protective effectiveness of forests against rockfalls, providing a valuable theoretical basis and practical guidance for the design of ecological prevention measures against rockfall hazards.