To get accurate knowledge of the real operating conditions of the transformer, different assessment methods are compared to identify the strengths, the weaknesses and the common law. First of all, the basic principles of the risk assessment and the transformer trust assessment are analyzed to find the advantages and disadvantages of each algorithm, and then the condition assessment of Yunnan Power Grid 50 transformers is conducted on this basis and the trust degree and security index are calculated. Finally, a comparative study is conducted and the threshold for condition-based maintenance strategy corresponding to the results of the assessment is identified. It will provide a more comprehensive and effective assessment basis for the transformer condition-based maintenance, and has an important guiding significance for economic and safe operation of the power grid.