Considering that the majority of Chinese language learners today have a certain level of proficiency in English, it is significant to conduct the research to explore the multilingual mastery of Chinese native speakers in the context of English as a second language. This paper explores the effect of native-based language competence on multilingual learners of Chinese by means of a summative scale questionnaire. This paper also explores the concrete language transfer of English as a source language through interviews and a specific analysis of compositions offered by six Chinese-English-German trilingual learners. The final conclusion is that when a native Chinese speaker learns a third language, the source language for language transfer will not be limited to Chinese. English as L2 has an impact on the learning and the use of vocabulary and grammar in L3, as well as on L3 writing, which both facilitates and hinders the learning and the use of L3. The influence of L2 on L3 is the most obvious at the intermediate level of L3 and learners with an advanced level of L3 are rarely affected by English. When L3 is an Indo-European language, it is more likely to be influenced by L2 English than by L1 Chinese.