It is not illegal to use the national flag as WeChat avatar. According to the provisions of the National Flag Law of the People's Republic of China, the National Flag cannot be used under the following circumstances:
 No damaged, defiled, faded or substandard National Flag may be displayed or used, and no National Flag may be displayed or used upside down, upside down, or in any other manner detrimental to the dignity of the National Flag. The national flag shall not be discarded at will. The National Flag that is damaged, defiled, faded or substandard shall be taken back and disposed of in accordance with the relevant provisions of the State. After the end of a large-scale mass activity, the sponsor of the activity shall take back or properly dispose of the national flag used at the activity site. The National Flag and its pattern may not be used as a trademark, a design for which patent rights are granted, or a commercial advertisement, or in inappropriate circumstances such as private funeral activities.
 Used as avatar is obviously not included in this list
 Legal basis
 Article 9 of the National Flag Law of the People's Republic of China, the State advocates citizens and organizations to use the National Flag and its patterns in appropriate occasions to express patriotic feelings. When citizens and organizations use the design of the National Flag on the Internet, they shall abide by relevant regulations on network management and shall not harm the dignity of the National Flag. The standard version of the national flag pattern used on the Internet is published on the website of the People's Congress of China and the Chinese government.