Unethical pro-organizational behavior (UPB) refer to employee’s unethical behavior for the benefits of the organization. In the past, many researches take it as the result of positive social exchange relationship and organizational identification. However, since that UPB is an unethical behavior in itself, some factors which had proved can influence unethical behavior should also influence UPB. In this paper, we explore the relationship between ethical climate and UPB, and the mediating effect of moral justification. We use longitudinal way to collect data from several organizations at two different time. The empirical results of 405 samples showed that egoism-, benevolence-ethical climate has a positive effect on moral justification and UPB respectively, principled-ethical climate has a negative effect on moral justification and UPB. Moral justification has a positive effect on UPB. Moral justification has a mediating effect between ethical climate (egoism-, benevolence-, principled-) and UPB. This research has several theoretic contributions and management inspirations. It is the first research to discuss the influence of ethical climate on UPB, and the results can expand the antecedent variables of UPB. We also find the mediating effect of moral justification between ethical climate and UPB, so it reveals moral disengagement is an important psychology formation of UPB. It also reveals that benevolence-ethical climate has “dark” side, and UPB has some instrumental motivations. For managers it is necessary to build principled-ethical climate, reduce egoism- ethical climate, and care about the dark side of benevolence-ethical climate. Monitoring and prevent individuals’ propensity to moral justification is also very important.