The invention discloses a method for analyzing and calculating withstand voltage of a composite insulator. The method mainly comprises the following steps: in a statically-stable or constant electric field of an insulator, establishing a Laplace's equation based on a withstand voltage analysis process of the composite insulator to be tested; based on the established Laplace's equation, solving a functional extreme problem and a variational problem which are based on definite solution problem equivalence in the withstand voltage analysis process of the composite insulator to be tested; based on solving results of the functional extreme problem and the variational problem which are based on the definite solution problem equivalence, solving potential of each node of the composite insulator to be tested. Through the adoption of the method, defects of high operating difficulty, narrow application range, high cost and the like in the prior art can be overcome so as to achieve advantages of low operating difficulty, wide application range and low cost.