ABSTRACT The energy backflow phenomenon is unavoidable in small dc‐link capacitor‐based PMSM drive systems, which is first identified and comprehensively analysed in this paper. This phenomenon occurs when the amplitude of the stator voltage exceeds the minimum value of the fluctuating dc‐link voltage. To address this issue, flux weakening (FW) control can effectively reduce the stator voltage and mitigate the energy backflow phenomenon. However, dc‐link voltage fluctuation can introduce several challenges to the conventional feedback FW control. For instance, a dc offset in the calculated d‐axis reference current is identified in this paper due to the dc‐link voltage fluctuation. Additionally, an analysis of the small‐signal model reveals that when the fluctuating q‐axis voltage becomes negative, it can lead to system instability. Moreover, the commonly used PI controller in FW control fails to adequately control the ac component introduced by these fluctuations. Therefore, this paper proposes an optimised FW control method to mitigate the energy backflow issues. Furthermore, an optimal phase angle selection method for the d‐axis reference current, based on the least mean square algorithm and gradient descent algorithm, is introduced to suppress current ripple caused by the fluctuating dc‐link voltage. Experimental results validate the effectiveness of the proposed optimised methods.