In real-world scenarios, cross-domain slot filling in spoken language understanding remains a significant challenge due to data scarcity. Previous works exhibit limited generalization ability in the target domain, demonstrating effective knowledge transfer only on seen slots while performing poorly on unseen slots. Although large language models (LLMs) can alleviate this issue to some extent, they underperform on seen slots compared to small models. To address these challenges, we introduce a novel framework that harnesses the power of a small model to augment the inferential capabilities of LLMs without additional training. Initially, we utilize target domain samples synthesized by LLMs as pre-calculated demonstrations, which are curated and chosen using confidence metrics derived from a small model. We further extract slot predictions from the small model to fully exploit its robust learning of familiar slots. Finally, during the inference process for test inputs, we integrate these demonstrations and slot prediction insights as references to enhance the slot filling performance of LLMs. Experiments on a slot filling dataset and a NER dataset including eight cross-domain settings show our framework achieves the best results. Our codes are publicly available at https://github.com/SIGSDSscau/SLSF.