This paper presents a novel sizing and optimization approach for the emerging serieshybrid unmanned convertiplane,which can be used to translate the top-level design requirements into the design parameters corresponding to the optimal power supply strategy and minimum total takeoff weight.The method comprehensively considers the design constraints in the rotor,fixedwing,and transition modes,and pays special attention to the characteristic response of Series Hybrid Electric System (S-HES) in complex application scenarios,especially the coupling of battery power,energy,and state-of-charge under high-power discharge conditions,the variation of fuel economy,and the adjustment of power supply strategy.With proposed method,it's possible to rapidly explore the design space in the initial design stage and find out the optimal design results with high confidence.A case study was proposed to verify the approach.The results reveal the particularity of convertiplane in terms of power requirements,and prove the necessity to consider detailed S-HES characteristic responses during parameter determination.The optimal design parameters were obtained through the hybrid control parameter optimization,which verified the effectiveness of proposed method.Possible errors and corresponding correction methods were also presented.