Background: Criminal judgment and decision making has triggered great interests among legal and psychological researchers. It refers to the processes of decision making related to guilt verdict and penalty by juries or judges based on the facts of the crimes according to the provisions of the law. Previous findings demonstrate that extraneous factors, such as irrelevant emotions, physical features of criminal suspects, etc., could affect criminal judgments. It is possible that juries or judges gain a positive or negative impression on a criminal suspect from knowing the information about his and the victims’ personalities, family background, personal experience or occupations , all of which should be considered as extraneous factors and should not taken into account in the course of criminal judgments. However, it is unknown how these factors affect legal decisions and what mechanisms underlie the process. Method: In order to investigate how and why the impression of parties to a case (the perpetrators and the victims) has an influence on the penalty judgments. Legal students and non-legal professional students were asked to make penalty judgments on the case, each of which included the same criminal act but contains different descriptions of the personal traits of the perpetrators and the victims. After the judgment task, participants were asked to assess the emotional responses triggered by the criminal behaviors. Results: Results showed that when the impression on the perpetrators was negative, participants felt more anger and disgust, less sympathy towards the criminal behavior; in the meantime, they gave higher punishment scores, than when the impression on the perpetrators was positive. When the subjective impression on the victims was positive than when it was negative, lower-grade participants gave higher punishment scores. No effect of the impression on victims was found on the punishment scores among higher-grade participants. Neither did we find any differences of the intensity for each of the three emotions between the positive and negative impression of the victims. Legal professionals reported lower emotional intensity than non-legal professionals towards the criminal behaviors. However, the professional background didn’t exert any significant influence on the penalty judgments. In order to explore the mechanisms underlying the influence of subjective impression on criminal judgment, mediation analysis was carried out. Results showed disgust fully while anger and sympathy partly mediated the influence of the subjective impression of the perpetrators on the penalty judgments. However, no mediating effect of any emotion was found for the influence of the subjective impression of the victims on the penalty judgments. Conclusion: This study suggests that the subjective impression of perpetrators and victims has significant influence on judicial decisions either in legal or non-legal participants. Participants tended to punish more harshly when they hold a relatively negative impression on criminals. In the process of the influence of the subjective impression of the criminals on the judgments, anger , disgust and sympathy emotions were found to serve as intermediary factors. However, in the process of the influence of the subjective impression of the victims on the judgments, no mediation effects of any of the emotions were found.