The constant-frequency double-rotor generator has the potential to be used in the marine energy conversions, such as tides and marine current power generation system due to the advantages. However, there are two different rotational speed movement boundaries in the constant-frequency double-rotor generator, especially the inner moving part boundary between the inner wound rotor and outer permanent magnet rotor. This complicates the finite element analysis and greatly increases the analysis time. In this article, a new finite element analysis method based on double boundaries interpolation multiple reference frame is proposed. The principle of the interpolation multiple reference frame method is introduced, and the analysis and comparison with the traditional method are carried out. The analysis results show that the interpolation multiple reference frame method simplifies the preprocessing procedure, reduces computation time, ensures high calculation accuracy, and has the potential to be applied in all kinds of multiple mechanical port machines.