Due to the bidirectional field modulated effect, the dual-permanent-magnet-excited (DPME) machine relies on the countless airgap flux density harmonics with specific pole-pair numbers (PPNs) to produce stable torque or effective back electromotive force (EMF). Regarding the generation of back EMF, there is no doubt that the flux density harmonics which make a negative contribution will reduce the performance of DPME machine. However, to the best of our knowledge, there is very little research on the identification of field harmonics which play a negative role in the generation of back EMF. In this article, the partial-analytical-method (PAM) is proposed to quantitatively identify the field harmonics. First, the PAM is described in detail, and the ratio of contribution (RoC) is defined to assess the contribution of field harmonics to back EMF. Then, the PAM is applied to a 12/10 DPME machine. The identification results show that two of the five types of field harmonics, i.e., Type-II and Type-V, make a negative contribution to back EMF; the RoCs of which are -2.94% and -4.32%, respectively. In these two types of harmonics, the ones with PPN = 22 are mainly responsible for the negative contribution to back EMF, which needs attention at the outset of the design.