The hot-wire anemometer is a widely used instrumentation to determine flow velocity and to investigate flow quality. The main objective of this paper is to expand the application range of the hot wire by improving the measurement accuracy under non-calibrated temperature and pressure. According to the four kinds of heat transfer derivations, a new calibration method was carried out. Considering natural convection, heat radiation and heat conduction, and forced convection heat transfer, it can be found that the forced convection heat transfer plays a dominant role, and the main factor causing the change is the temperature. Forced convection heat transfer also changes with pressure, which affects heat transfer by affecting kinematic viscosity. Based on this, a new calibration method and formula of velocity were put forward, which can be used over a range of temperature and pressure, considering the changes of physical property of the calibration scheme were verified by numerical simulation. The numerical calculated results were compared, the average error was 0.69%, the maximum error was 2.9%. The results show that the calibration method has high accuracy in a certain range. This paper provides a new solution for the calibration of hot-wire anemometer, and expands the adaptability of hot-wire anemometer in the measurement of severe external conditions.