The fuelling efficiency and injection depth are the fundamental problems of supersonic molecular beam fuelling. In the recent supersonic molecular beam injection (SMBI) experiments, the injection depths of SMBI have been found mainly dependent on the background electron density and temperature, the source gas pressure and temperature. The empirical scaling of SMBI depth was obtained at room temperature. Hydrogen clusters formed in the beam have been observed in the low temperature (source gas at liquid nitrogen temperature) SMBI experiments. It resulted in injection depth greater than 30 cm. Comparing the relationships between injection depths, plasma background parameters and molecular source temperature, the cluster formation phenomena during low temperature MBI are analyzed.