The thermodynamic and calorific of the gas in wind tunnel will be imperfect when the wind tunnel operates under cryogenic temperature. In this case, the real gas effects should be considered for calculating flow parameters. The change laws of the compressibility factor and specific heart ratio for nitrogen gas under the ranges of pressures (100 ~ 450 kPa) and temperatures (100 ~ 323 K) were presented. By introducing the isentropic expansion coefficient into one-dimensional perfect gas flow equations, the computational analysis model for cryogenic wind tunnel flow parameters was developed, and the operating envelope of transonic wind tunnel considering the gas liquefaction temperature and pressure under high speed was obtained. The analysis results show that for the ranges of pressures (115 ~ 450 kPa) and temperatures (110 ~ 323 K) that cover the range of operation of transonic cryogenic wind tunnel, the maximum deviation of the various isentropic parameters from the ideal values is about 1% or less, and this deviation would be insignificant for the aerodynamic designing of the cryogenic transonic wind tunnel.