Abstract: In hypersonic low density nozzle flow, the vibrational temperature is frozen, causing serious non-equilibrium relative to the translational and rotational temperatures. In related numerical simulations, the vibrational temperature is completely frozen, but the experimental data is obviously lower than the simulation result. To address this issue, analyses have been performed on the numerical method in the applications of low temperature conditions. Based on the simulation method of thermal non-equilibrium used in direct simulation Monte Carlo (DSMC), by modifying the number of vibrational relaxation collision, a numerical method for vibrational non-equilibrium flow at low temperature is proposed, in which the modified coefficient is set to be 1×10⁻⁷. The flow in a M16 nozzle of the low density wind tunnel and the plume of 10 N attitude control thruster is simulated, and the vibrational temperature agrees with the experimental result. It is shown that, due to the large value of the number of vibrational relaxation collision in the low temperature condition, gas molecules can rarely have vibrational relaxation collisions, and the vibrational energy cannot transform to the translational energy, thus the vibrational temperature is completely frozen. With the present introduced modification, the number of vibrational relaxation collision is reduced, the probability of vibrational relaxation collision is increased, and the vibrational temperature can be reduced to match the experimental results.