Based on third language acquisition theories (Flynn, Foley, & Vinnitskaya, 2004; Marx & Hufeisen, 2004; Rothman, 2010, 2015) and cross-linguistic influence (Sharwood Smith & Kellerman, 1986), this study explored the written production errors of L3 English acquisition of word order in the affirmative and interrogative structures by L1 Yi and L2 Mandarin learners. The participants were thirty "Yi ethnic minority" students of L3 English at the beginner level from a middle school in Yunnan Province, China. The instrument was an elicited production task. The findings exhibited that errors in L1 Yi word order were more frequently produced than those of L2 Mandarin word order, and the errors produced in the interrogative structures were higher than in the affirmative structures. It demonstrated that cross-linguistic influence from L1 Yi and L2 Mandarin was more evident since a higher proportion of error rates from these two languages were examined, and L1 Yi tended to have a higher negative impact than L2 Mandarin.