This study concentrates on the study of optimisation-based tuning method of fractional-order proportional integral (PIλ or FOPI) controller, and the application of FOPI controller in permanent magnet synchronous motor servo drive system. The main contribution lies in two aspects: (i) A practical FOPI tuning method is proposed. The tuning method formulates a non-linear optimisation problem by maximising the closed-loop bandwidth with constraints on complementary sensitivity, and gain and phase margin. Compared with the existing FOPI tuning strategies, the proposed method is suitable for both integer order and fractional order control systems and, it fully considers the stability requirement of the servo system. (ii) An enhanced stochastic multi-parameters divergence-based optimisation (SMDO) algorithm is presented to solve the non-linear optimisation problem in the FOPI tuning. The enhanced SMDO has the advantages of easy to implement, effective for fast convergence to minima, stochastic in nature and simple adaption to real-time application. Compared with the traditional SMDO method, enhanced SMDO owns faster convergence speed and better global search ability. Experiments are presented to demonstrate the effectiveness of the proposed tuning method and enhanced SMDO algorithm.