To investigate the influence of L2 proficiency and syntactic similarity on English passive sentence processing, the present ERP study asked 40 late Chinese-English bilinguals (27 females and 13 males, mean age = 23.88) with high or intermediate L2 proficiency to read the sentences carefully and to indicate for each sentence whether or not it was correct. Sentences were classified into literal translation sentences with the similar structure between the two languages and free translation sentences with the different structure. Behavioral data showed: shorter reaction times and higher accuracy rates occurred in the high-proficient group than those in the intermediate-proficient group; shorter reaction times and higher accuracy rates were observed in literal translation sentences than those in free translation sentences. ERP results showed literal translation sentences elicited an enhanced P200 and P600 while free translation sentences elicited a larger N400. The high-proficient group showed a larger P600 in syntactic violations and double violations while the intermediate-proficient group evoked an enhanced N400 in semantic violations and double violations. Literal translation sentences caused a larger P200 while free translation sentences elicited more negative-going N400. Behavioral and ERP data revealed the influence of L2 proficiency and syntactic similarity on L2 sentence processing, and L2 proficiency played a predominate role.