The construction of subject-verb agreement in English involves complicated cognitive processing and thus often elicits mistakes in English production among Chinese learners of English. This experimental research, by adopting a two-choice response time paradigm, explores number attraction effect and the cognitive mechanism involved in English S-V agreement processing on the part of middle level English learners in Chinese universities. The results show that number attraction effect leads to the prolonged response time in correct judgment and the lower accuracy in S-V agreement processing, having strong effect size; the negative effect caused by collective head noun in the subject is neutralized in the presence of the number attractor compared with the cases of singular heard noun; and the students at the middle English level incline to process English information in a serial way instead of the hierarchical way .