It is illegal to sign the national flag. According to the criminal law, this act is suspected of insulting the national flag. A natural person who commits this crime shall be sentenced to fixed-term imprisonment of not more than three years, criminal detention, public surveillance or deprivation of political rights. But insulting the national flag must reach a certain level to constitute a crime.

legal ground

Article 299 of the Criminal Law

Whoever intentionally insults the people of China and the national flag and emblem by burning, damaging, scribbling, defileing or trampling on them in public places shall be sentenced to fixed-term imprisonment of not more than three years, criminal detention, public surveillance or deprivation of political rights.

Whoever intentionally falsifies the lyrics and music scores of the China people's national anthem in public, plays the national anthem in a distorted or derogatory way, or insults the national anthem in other ways, if the circumstances are serious, shall be punished in accordance with the provisions of the preceding paragraph.