Through the operation experience and statistical analysis, the state variables of detection can be got without cutting power, which can effectively reflect the condition of power transformer. Based on the morphological theory, this paper presented a hybrid filter for on-line monitoring dates which was combined with the open-closing filter and close-opening filter. The evaluation function was established by eighteen state variables which were got without cutting power, such as partial discharge, oil chromatography, casing leakage current, casing dielectric loss factor and so on. A multilevel evaluation model was also proposed based on the evaluation of state variables, parts and the whole of transformers, which could be used to evaluate the condition of working transformer.