The present study investigated the relative importance of phonological short-term memory (PSTM) and lexical knowledge for second language (English) vocabulary learning in Chinese-English bilinguals. To clarify the conjoined influence of two factors, we distinguished PSTM into serial order STM (measured by serial order reconstruction task) and item STM (tested by delayed single nonword repetition), and nonwords were manipulated in phoneme familiarity (English nonwords and mixed nonwords). Participants are all Chinese native speakers who learn English as a second language with medium proficiency. The paradigm of picture-nonword pairs was adopted in this research. Using a learning way of auditory presentation and speech output, Experiment 1 demonstrated that nonword repetition and lexical knowledge are two independent predictors of familiar nonword learning; as for unfamiliar nonwords, lexical knowledge and serial order STM measurement independently predicted its performance. Since the results of Experiment 1 can be caused by learning difficulties and inaccuracy of pronunciation, picture recognition in the paired-associate learning were used in Experiment 2 to explore this question further. The results of Experiment 2 show that serial order STM, item STM and lexical knowledge are all independent predictors of familiar nonword learning, and serial order STM and item STM can also predict the performance of unfamiliar nonword learning independently. These results illustrate the important role of PSTM and lexical knowledge in second language vocabulary learning among Chinese-English adults, and the influence would varied since the characteristics of input information and ways of learning are different.