Solution:
First, the subtitle files must be in the same directory as the video files. Secondly, the name of the subtitle file should fully contain the name of the corresponding video file, for example, the name of the video is "Avatar. mkv", then the filename of the external subtitle should be "Avatar XXX.srt". ".mkv" and ".srt" are suffixes for the video and subtitle respectively, and XXX can be any legal character.