With aiming at the problem of liquid hydrogen flowing through filling system in space launch site, the author set up a math model of laminar flow and coupled with the analysis of laminar flow condition of liquid hydrogen in tube. Based on the computational fluid dynamics, the flow field was studied according to the numerical simulation method. Then, the changes of flow parameters i.e. pressure and velocity at the tube were observed. Simulation results showed that, in the inlet-section, there existed boundary-layer, besides, the velocity and pressure which changed continually was not stable until the liquid hydrogen flowed through on-way. In addition, the velocity distributed in the form of rotary parabolic surface whose axis was the tube’s axis itself in water-carrying section of on-way. The velocity near the surface of the tube was about zero and the pressure turned to smaller with the increase of tube length. As a result of the pressure drop, there existed the loss of on-way. The above simulation results which provided the theoretical basis for liquid hydrogen study accorded with the actual condition.