There should be two witnesses who have nothing to do with the case at the time of appraisal, and the record of appraisal should be signed or sealed by the participants and witnesses. The necessity and number of witnesses are limited. Article 4, paragraph 3 of the Rules of the Ministry of Public Security for On-the-spot Investigation of Criminal Cases by Public Security Organs stipulates that on-the-spot investigation must invite two impartial citizens who have nothing to do with the case as witnesses, and public security judicial personnel cannot be witnesses.