Can I write a letter of understanding in criminal cases?

Of course. In a specific case, if the parties agree to write a criminal understanding, as long as the content conforms to the law and the parties voluntarily and legally sign it, the criminal understanding will have legal effect.

Legal basis:

Article 288 of the Criminal Procedure Law of People's Republic of China (PRC), in the following public prosecution cases, if the criminal suspect and the defendant sincerely repent by means of compensation for losses, apology, etc., and the victim voluntarily reconciles, the two parties may reconcile:

(a) due to a civil dispute, suspected of criminal cases stipulated in Chapters IV and V of the Specific Provisions of the Criminal Law, and may be sentenced to fixed-term imprisonment of not more than three years;

(two) criminal cases of negligence other than dereliction of duty that may be sentenced to fixed-term imprisonment of not more than seven years.

If a criminal suspect or defendant intentionally commits a crime within five years, the procedures specified in this chapter shall not apply.