With the ongoing advancement of science and technology, the multimedia teaching method is becoming more and more important. Among them, watching movies and TV plays is the most common way because it can provide a relaxed learning environment. This paper attempts to explore how subtitles in videos can assist learners to acquire a second language, whether positively or negatively. Since the author is from China, the first language mentioned in this paper basically refers to Chinese and the second language refers to English. In the course of this research, the author reviewed the past literature and experiments from China and abroad, and also combined them with her own experience in the process of watching videos. Finally, a conclusion is reached that in general, applying subtitles to videos like movies or TV series is helpful for the whole second language acquisition, but in different categories of subtitles (L1, L2 or both L1 and L2), learners' second language competence and different types of videos also have positive and negative effects on it. In response to this conclusion, this paper also puts forward corresponding suggestions to different groups, such as teachers, learners, media, video producers, etc. at the end of the paper. The aim of this paper is to look forward that in the future, the method of watching videos can be more beneficial for acquiring a second language.