The expansion and transportation of supersonic molecular beams is a complex process of molecular dynamics, and the related parameters are difficult to calculate accurately. Currently there is no rigorous theory to accurately predict the beam expansion process under specific valve conditions, and current researches are less concerned with the spatial evolution of supersonic molecular beam characteristics over long distance. In addition, time-of-flight mass spectrometry is not well suitable for supersonic molecular beam injection in the field of magnetic confinement fusion. Therefore, based on microphone measurements, the average velocities of several supersonic molecular beams (H ₂, D ₂, N ₂, Ar, He, CH ₄) in the process of free expansion and their evolutions in the far-field space (flight distance/nozzle diameter > 310) are studied in this work. The variations of velocity distribution with gas type, temperature, pressure and expansion distance are obtained. The results show that the velocities of H ₂, D ₂and H _ebeams account for only 54%, 60% and 68% of their ideal limit velocities, respectively, and their velocities decrease rapidly in the far-field space. The velocities of CH ₄, N ₂and Ar beams are very close to their limit velocities, accounting for 85%, 92% and 99% respectively, and their velocities decrease slowly in the far-field space. And the results show that the velocities of the H ₂and D ₂beams increase with the source pressure, while the velocities of the other four molecular beams decrease slightly with the source pressure. And it is found that the velocity of supersonic beam without skimmer is negatively correlated with the square root of the molecular mass. For the effect of temperature on velocity, the results show that the velocities of H ₂and D ₂beams increase with the source temperature but are smaller than their limit velocities at given temperature, and the difference is larger for higher temperature. The results of this experiment provide basic data for controlling the parameters of the supersonic molecular beam by adjusting the temperature and pressure of the gas source, which will contribute to the application of supersonic molecular beams in fusion reactor fueling technology. And this study will contribute to further exploration of the evolution of supersonic molecular beam properties in the far-field space.