This research was conducted with a view to investigating the effects of Length Approach on L2 vocabulary learning. The study employed both a quantitative and a qualitative study, which yielded the following findings: with regard to college English vocabulary learning, learners’ productive ability lags far behind their receptive ability. Learners’ capability of using words in communication does not develop correspondingly with their vocabulary size. Learners tend to learn English words by means of studying the word list, which may be very dangerous, since what it displays is just pronunciation, word class, target-language equivalents, derivatives and occasionally one or two examples. Long composition writing task leads learners to become more aware of the new expressions or lexical phrases which can help enrich the content of their writing. The requirement of Length Approach is helpful to arouse learners’ consciousness to accumulate new words and expressions and then use them in their writing. The data from the experiment provides supporting evidence to prove that exposure to meaningful comprehensible output practice enables learners to improve a lot in terms of language productive proficiency. The results demonstrate that Length Approach plays a positive role in building-up students’ confidence, developing their initiatives, arousing their interest and motivating them to use more new words and expressions in writing practice, and enhancing their ability in English vocabulary learning. Length Approach, through awakening a producing desire and awareness in learners, promotes the transition from input to intake by timely practice of long composition writing.