Mixed cultivation of Larix gmelinii and Juglans mandshurica is a typical strategy for increasing stand productivity in Northeast China. However, the adaptive strategies of fine roots and root-associated fungi (RAF) after mixed cultivation remain unclear. Here, we examined the chemical, morphological, and anatomical characteristics of fine roots, along with the composition, diversity, and co-occurrence network structure of their RAF communities. Our results showed that mixed cultivation increased the root diameter and root tissue density of first-order to third-order fine roots for both Larix gmelinii and Juglans mandshurica, but decreased the specific root length. The root economic spectrum of the two species demonstrated a shift from a “do-it-yourself” strategy to an “outsourcing” strategy in their first- and second-order roots after mixed cultivation. Arbuscular mycorrhizal fungi and endophytic fungi were the main fungal functional groups within the RAF of Juglans mandshurica, while ectomycorrhizal fungi were dominant in those of Larix gmelinii. Mixed cultivation increased the RAF alpha diversity of Juglans mandshurica but decreased the RAF alpha diversity of Larix gmelinii. Negative correlations in the co-occurrence networks of the RAF communities accounted for more than 50% of the two species, indicating that competitive relationships dominated within the RAF community. Changes in the composition of RAF after mixed cultivation effectively supported shifts in the root economic spectrum of the two species. The coordinated changes in fine root systems and their associated mycorrhizal fungi enable the two species to maintain their competitive edge in nutrient absorption when they are planted together.