High-speed train gearbox lubricant provides lubrication not only for high speed gear, but also for support bearings at both ends, so the selection of its viscosity index requires a more rigorous analysis for lubrication law. The lubricant viscosity at 40 ℃ and 100 ℃ of CRH380A high-speed train gearbox were calculated with the mechanical design experience formula, viscosity empirical formula, elastic fluid lubrication analysis method and elastic fluid lubrication empirical formula， the agreeable viscosity index was got, which provides practical models and theories for the selection of high-speed train gearbox lubricant. The research results show that, because of the large surface contact stress, high temperature and large circumferential velocity, highspeed train gearbox lubricant should have high abrasion resistance, thermal stability, oxidation stability, and the capability to operate in a wide temperature, and can provide good lubricating and security properties on gearbox bearings.