A ferrocene-assisted calcination process (FACP) has been applied during the preparation of LiFe0.6Mn0.4PO4/C cathode material. Ferrocene works as a promoter to improve the quality of the coated carbon layers. The influence of ferrocene on the property of carbon layers and electrochemical performance of LiFe0.6Mn0.4PO4/C has been investigated systematically. The LiFe0.6Mn0.4PO4/C with FACP presents more uniformly and tightly coated carbon layers with higher graphitization degree than the sample without FACP. High-quality carbon layers can greatly enhance the electrochemical performance of cathode materials. The LiFe0.6Mn0.4PO4/C with FACP behaves higher discharge capacities (163.6 and 120.7 mAh g−1 at 0.2C and 20C, respectively) than the LiFe0.6Mn0.4PO4/C without FACP (158.4 and 111.5 mAh g−1 at the same rates). Furthermore, the LiFe0.6Mn0.4PO4/C with FACP exhibits excellent cycling performance even at 60 °C, still delivering capacity retention above 96% after 100 cycles at 3C, while the retention of the controlled sample is only 70% after 75 cycles. The cycling performance of the LiFe0.6Mn0.4PO4/C with FACP at high temperature is largely improved.