To improve the performance and explainability of conversational recommender systems (CRS), we propose the COMPASS framework, which combines LLM and knowledge graphs (KGs). COMPASS pre-trains graph entity captioning to bridge the gap between KG information and natural language, and optimizes user preference inference through knowledge-based command fine-tuning. This allows COMPASS to generate interpretable user preferences and integrate them into existing CRS models, improving recommendation performance and explainability.