Thermodynamic probability is an important concept in statistical physics, and relates to the Boltzmann entropy directly. In this paper, factors governing thermodynamic probability are analyzed, which may be neglected in calculation, i.e., the state of ideal gas imposes restrictions, and ensures the Boltzmann entropy an extensive quantity. The thermodynamic probability change of the following processes of ideal gas is calculated, i.e., adiabatic free expansion, isothermal expansion and varying molecular number under isothermal and isobaric conditions.