With the rise of 6G communication technology, the issue of communication coverage in mountainous areas has become increasingly prominent. These regions are characterized by complex terrain, sparse user distribution, and small-scale clustering, making it difficult for traditional ground-based base stations, constrained by fixed locations and terrain obstructions, to achieve comprehensive signal coverage in mountainous areas. To address this challenge, this paper conducts an in-depth analysis of mountainous terrain and the differentiated needs of users, utilizing UAV-assisted base station signal coverage and designing an adaptive multi-scale bidirectional twin delayed deep deterministic policy gradient (AMB-TD3) algorithm to optimize base station layout and plan UAV routes. The algorithm significantly enhances performance by introducing a dynamic weight adaptation mechanism, multi-timescale coupling, and bidirectional information interaction strategies. In experiments, the best signal coverage rate of AMB-TD3 reached 98.094%, verifying its practicality in solving base station signal coverage issues in complex mountainous scenarios.