The trust management mechanism plays an important role in protecting the security of Underwater Acoustic Sensor Networks (UASNs). When trust evaluation is performed for a certain target node, it is usually necessary to integrate recommendations of third-party nodes to complete the trust decision. However, when the recommendations of multiple third-party nodes conflict with each other, the trust decision will become complicated. In this paper, we propose a lightweight Trust Management mechanism based on Conflict Adjudication (TMCA) for UASNs, analyze the causes of conflict and propose corresponding solution. First, trust evidences are collected and used to evaluate trust of the target. Then, node reliability and link reliability are defined to address the recommendation conflict in the process of trust evaluation. Simulation results demonstrate that the proposed TMCA outperforms traditional algorithms in terms of trust evaluation accuracy and malicious node detection.