Considering the influence of the heat transfer and friction between the dilute gas-particle flow and the inner wall of lance, a mathematical model describing the behavior of the dilute gas-particle flow passing through constant-area lance has been developed. According to the Mach number checkout method, the choking phenomena for heat and friction of dilute gas-particle flow with the lance wall were determined. At the same time, the critical duct length of gas-particle flow was acquired by the model.