This report is based on the statistical data of the 2020 Internet Use by Minors in China survey. The researcher conducted in-depth interviews with left-behind children，their guardians and teachers from January 2021 to March 2021 in various places in China. Research on the Internet usage of left-behind children finds：their own cell phones have become the preferred device. Their age of first internet use is low，and they spend too much time online. They use a variety of devices. The supervision of their internet use is insufficient. Video is the most popular application. Most of them have pre-paid for services.The report offers recommendation for found from government，school，family and society.