Abstract: The power equation of double-stator permanent magnet brushless （DS-PMBL） machine had been derived and its output power features had been analyzed under different magnetic loadings.Accordingly,the mathematical expressions of optimal split ratio,namely the ratio of inner-stator outer diameter to outer-stator outer diameter,had been deduced,from which the maximum output power could be obtained.Then the reasonable power distribution between inner and outer airgaps had been discussed quantitatively.At last,the split ratio of a prototype DS-PMBL machine had been optimized in order to achieve maximum power density.Both finite element analysis and experimental results of the prototype machine were given to verify the design method and the theoretical analysis.