In the case of using the molecular dynamics method to simulate the micro-canonical ensemble, taking the monoatomic particle system as an example, the mathematical expression of the scaling factor under the International System of Units is given. Then it explains how to rescale the physical quantity to obtain the dimensionless formula, which makes the calculation more convenient in the simulation. The physical meaning of the time unit in the dimensionless operation is given. Finally, the mathematical expressions commonly used in textbooks for the speed scaling factor of the balance process are derived.