The working load and stroke of energy absorption are two key performance parameters for energy absorber in designing crashworthy seat. A design method for expandable tubular energy absorber (ETEA) was proposed, which specifically involves the design methods of performance and structural parameters of ETEA. Firstly, the working load of ETEA was determined through establishing its relationship with the lumbar load of occupant from the perspective of biomechanics. Secondly, the stroke of energy absorption of ETEA was derived based on the mathematical model of helicopter crash. Thirdly, the relationship between the working load and the structural parameters of ETEA was determined based on the principal stress method, and the structural parameters of ETEA was obtained through clarifying the determined design parameters. Finally, the design method proposed was implemented by taking a helicopter seat as an example, and the expanding deformation and loading process of ETEA were investigated through numerical simulation and experiment. The results show that the magnitudes of working load in numerical simulation, experiment and design method are very close, demonstrating that the design method proposed is feasible and effective, and could be provided as theoretical guidance for the design of crashworthy seat of civilian helicopter.