The perfective aspect marker in Chinese is partly functionally similar to inflectional suffixes in Indo-European languages but is non-inflectional and lexical in nature, lying thus at the semantics-syntax interface. This provides us with the opportunity to compare directly the syntactic and semantic constraints during second language (L2) sentence processing. The present study explored how L2 Chinese learners with Indo-European languages as their first languages (L1s) process the Chinese perfective marker. The Competition Model prioritizes syntactic processes entailed by cross-linguistic transfer from the participants' L1s, but this prediction might be challenged by the concurrent functioning of semantic processes. In an event-related potentials (ERP) experiment, 22 European language-speaking L2 Chinese learners with low to intermediate proficiency level and 20 native Chinese speakers (i.e. the control group) participated. An aspectual agreement paradigm was used for materials. Results showed that in the aspect marker mismatch condition, L2 Chinese learners with a shorter learning experience were more likely to show a P600-like component, indicating a morpho-syntactic routine, supporting thus the predictions of cross-linguistic transfer based on the Competition Model. Those with a longer L2 learning experience were more likely to show an N400-like component similar to native Chinese speakers. This shift from P600 to N400 for more advanced learners suggests that L1-L2 syntactic similarity may exert much stronger influence than semantic constraints for learners with shorter L2 experience.