The flow behaviors under different filling pressures were observed using electrode contact method. The relationships of recorded time and related point location were analyzed and deduced in terms of the flow tendency. The flow behavior figures were plotted at different time according the relationship. It was found that the unstable flow pattern takes place because filling cross-sectional area changes abruptly from the flow behavior figures. According to energy conservation and mass conservation theory, the relationship of flow velocity and filling pressure was established. Combining this relationship and critical flow velocity, the judgment equation of critical filling pressure was achieved.