Aiming at the problem that the traction winch bearing the main load of the tethered balloon system lacks the design theory, the tension release principle of the traction winch was described. The differential method and the stack multiplication method were used to construct the mechanical model of the winch.The law and mathematical equation of tension attenuation were obtained, and the attenuation coefficient of the cable tension was analyzed.The results show that the effect of cable tension attenuation is only related to the total cable wrap angle α, the equivalent friction coefficient μ and the centrifugal tension Tc;the larger the α and μ, the better the tension attenuation effect, and the excessive α and μ will reduce the cable life and aggravate the uneven load of winch;a larger Tc will reduce the attenuation effect, but it will be beneficial to the balanced load of the winch; reasonable attenuation coefficient is conducive to the uniform release of cable tension. According to the analysis results, the structure design and finite element simulation of the winch were completed. The research has certain engineering guiding value for the design of the traction winch