<p>This study conducted two experiments to investigate the influence of vocabulary difficulty and retrieval interval on the retrieval practice effect in English-language acquisition. Experiment 1 involved 35 non-English-major college students who were tasked with learning 20 easy English–Chinese word pairs and 20 difficult English–Chinese word pairs through retrieval practice and re-study methods. Results from the delayed test conducted two days later indicated that, regardless of vocabulary difficulty, retrieval practice consistently yielded better performance than re-study. Furthermore, the results indicated that test performance was significantly lower for difficult word pairs than for easy word pairs. Subsequently, Experiment 2 was conducted to investigate the impact of retrieval interval on the learning of challenging English vocabulary, using the difficult English–Chinese word pairs from Experiment 1 with varying retrieval intervals. The delayed test administered two days later revealed that both equal-interval retrieval practice and expanding-interval retrieval practice were effective in improving test performance for difficult English vocabulary. In conclusion, our study demonstrates the consistent presence of a retrieval practice effect in the learning of English vocabulary of varying difficulty. Furthermore, retrieval practice performance for difficult vocabulary was significantly lower than for easy vocabulary. Interval retrieval practice effectively promotes the learning of challenging English vocabulary. Both expanding and equal intervals facilitated the acquisition of difficult English vocabulary.</p>