The power transformers are very important in the power systems. Reliability modelling of power transformers has a great impact on the design and security of the grid. In this paper, we assess a reliability model of power transformers using different MarKov groups , we divide the power transformers into three parts which are series system, wingding system and maintenance system. In order to get the reliability assessment, We consider the impact on the reliability of power transformer which wingdings working in an derated state make. Then, according to the statistics on reliability indices of power transformers and the changing proportion of windings in derated state, we assess the reliability indices of power transformers combining the steady-state probabilities in transformers and the frequency variation. The simulation shows that the method is valid and reliable which can provide the operator more theory support.