The invention provides a power transformer risk assessment method based on a fault tree. The power transformer risk assessment method comprises the following steps that 1) according to the given fault tree, fuzzy judgment matrixes R are built by using the fuzzy analytic hierarchy process; 2) according to the fuzzy judgment matrixes R, a constraint programming problem equation set is written out, wherein the severity weights omega i of various fault causes are obtained according to the constraint programming problem equation set, and the severity weights vector W satisfying the equation that W=[omega 1, omega 2,...omega n](T) is obtained; 3) according to the severity weights vector W, the optimal solution of the severity weight omega i enabling an adaptive value Z to be minimum is obtained; 4) the optimal solution of the severity weight omega i is substituted into the formula (4) to obtain the risk coefficients of the fault causes of a transformer. According to the power transformer risk assessment method, matrixes achieving fuzzy judgment consistency do not need to be built, and risk of the transformer can be assessed under the circumstance that the fuzzy judgment matrixes are not consistent; compared with the method in which the matrixes achieving fuzzy judgment consistency have to be built in the prior art, the power transformer risk assessment method is simple in algorithm, rapid and relatively high in precision.