Working memory and interpreting expertise are generally acknowledged as two essential factors that affect the interpreting process. With reference to the compensation model, the independent influences model, and the rich-get-richer model proposed in cognitive psychology concerning the interplay between working memory and domain expertise, this experimental study, by varying the delivery speed of the source speech, examines the interplay between working memory and interpreting expertise in the setting of simultaneous interpreting. This study finds that (1) working memory and interpreting expertise can each have a positive impact on simultaneous interpreting quality; (2) other factors being equal, interpreting expertise can have a greater impact on simultaneous interpreting than working memory, and when the interpreting task proves more challenging, interpreting expertise can help relieve the pressure triggered by inadequate working memory; (3) working memory and interpreting expertise function differently in simultaneous interpreting in different stages: for beginner interpreting students, working memory’s contribution to the quality of simultaneous interpreting is more prominent, while for advanced interpreting students, interpreting expertise plays a more direct and prominent role, and the composite influence of working memory and interpreting expertise on simultaneous interpreting performance is weakened due to the increasing impact of other factors.