The research investigated the influences of English derivational affixes on EFLs' lexical access through an on-line semantic decision task. The manipulation of variables came to a 3 (word formation: non-derivatives, prefixed derivatives, suffixed derivatives)× 2 (semantic relatedness: semantically related, semantically unrelated)× 2 (L2 proficiency: medium level proficiency, low proficiency). The result showed that 1)The affixes had significant inhibitory effects on the lexical access of derivatives, and the inhibition of prefixes was significantly higher than that of the suffixes, suggesting that the decomposition process might have occurred during the semantic processing;2)The inhibitory effects of suffixes grew weaker with the progress of L2 proficiency; nevertheless, the inhibition of prefixes were still very significant even for the medium level proficiency participants. The research result was aimed to provide more data and evidences for the research of semantic processing and acquisition of English words with complex morphological structures.