This paper proposes an evaluation index for the analysis of the motion complexity in parallel multi-degrees-of-freedom (MDOF) haptic system. Firstly, the modal decomposition based on discrete Fourier series expansion (DFS) is described. Modal information expresses a motion element that corresponds to a specific physical action. The modal information can be defined by the Fourier coefficients. This paper proposes a total harmonic distortion (THD) of the haptic modal information as a haptic motion index. The utility of the proposed index is confirmed from the experimental results on the bilateral motion control of MDOF haptic system.