Supercooling phenomenon of stratospheric long duration balloons means that the temperature of helium gas inside is lower than that of atmosphere outside, this will cause the loss of buoyancy and affect ascent process of the balloon as a result. Radiation and convection thermal models and the vertical dynamic model of long duration balloons were established, and the influence regularity of some important factors, including initial net buoyancy, skin thermal physical parameters, launch time and launch date, on supercooling phenomenon was analyzed. Simulation results show that the temperature difference between helium gas and outside atmosphere enlarges remarkably along with the increase of initial net buoyancy, and the temperature difference decreases when the absorptivity for visible light and infrared radiation increases. Simulation results also show that the influence of launch time and launch date on supercooling phenomenon is much smaller, while flight time from ground to designed altitude displays great difference when the launch time is different. The research results can provide theoretical reference for conceptual design and flight test.