In recent years, the application of complex network methods in linguistic research has emerged as a new approach within digital humanities. This study analyzes the topological features of interlanguage syntactic dependency networks to explore the different roles of the high-frequency words “是”(shi) and “的”(de) in the interlanguage of both beginner (A1) and advanced (C2) Japanese CFL learners. By examining the changes in the network after removing these two words, we found that “是”(shi) serves as an important bridge in beginner interlanguage. Its removal results in an increase in average path length and a significant decrease in betweenness centralization, reflecting the strong dependency of beginner learners on “是”(shi) Conversely, the dependency on “是”(shi) is relatively weaker in advanced interlanguage, with its removal having a smaller impact on the network structure. On the other hand, “的”(de) functioning as a modifier node, leads to more direct connections between other nodes upon its removal. This is particularly evident in the advanced network, where it significantly enhances centralization, reflecting the flexible organization of grammatical structures by advanced learners. The findings reveal the distinct roles of “是”(shi) and “的”(de) in interlanguage networks, offering new insights into the syntactic organization and developmental process of interlanguage.