Legal basis: Article 306 of the Criminal Law of the People's Republic of China, the crime of perjury by defenders and agents ad litem is a sharp sword hanging over the head of a lawyer. This article stipulates that "in criminal proceedings, defenders and agents ad litem destroy or falsify evidence, help parties destroy or falsify evidence, threaten or induce witnesses to change their testimony or give perjury that goes against the facts", which can constitute this crime. Once a party or witness makes a different statement during a trial than before, it is likely to cause dissatisfaction on the part of the party, and the party or witness may be forced to change their statement again through coercion, inducement and other means. In this case, today’s criminal lawyers will rarely investigate and collect evidence themselves. One is that their ability to collect evidence is limited, and the other is that they dare not collect evidence.