Based on the thorough analysis of typical failure modes and influences of on-load tap-changers in power transformer, the fault-tree of tap-changers is set up using fault-tree analysis techniques and the assessment method for the reliability of tap-changers, combined with the index of the importance and the weight integration method based on the multi-objective decision-making theory and subjective and objective integrated weights, is put forward. The assessment examples show that the method can deduct and ratiocinate the failure modes of tap-changers clearly, find out the weaknesses in time during the operation, reflect the influence degree of each fault modes, obtain the reliability level of tap-changers in the operation and get better assessment results.