The degree of collocational knowledge influences learners' competence in the production of L2 speech and text. However, collocation learning is complex because learners might co-produce words incorrectly according to L1 inference. This study aims at the design of an online video-assisted collocation learning system, VACLS, that integrated video captioning and concordancing to foster collocational knowledge acquisition. English collocations with three patterns, namely verb-noun, verb-adverb, and verb-preposition, were selected as target collocations in VACLS. While learning with VACLS, the learners are able to watch videos with full captions and then looked for additional knowledge and instances of the target collocations via an online concordancer incorporated in VACLS. Besides the development of VACLS, effectiveness of the system and learners' perception were also evaluated. 20 university EFL students in Taiwan enrolled the experiment. After 3 weeks of learning with the aid of VACLS, the learning outcome for collocation retention was significant and the participants with less initial collocational knowledge exhibited greater improvement in collocation retention after learning VACLS. Besides, these learners highly agreed VACLS is helpful for English collocation acquisition. Thus the VACLS can be useful for English educators and EFL/ESL learners.