Abstract: A systematic investigation on the causes of MWD signal dropout after total lost circulation in directional drilling in low-pressure formations has yet to be fully established. Wherefore, based on the U-tube principle of drilling fluid circulation, a model was established to calculate pump pressure, flow resistance inside and outside the drillstring, and annular fluid level height under conditions of total lost circulation. A case study of a certain well in Tongchu was conducted here. The annular fluid level height was calculated from changes in pump pressures. The variation patterns of pump pressure and annular fluid level with respect to circulation rate, density, viscosity, and downhole tool flow resistance were analyzed qualitatively and quantitatively. The pressure distribution within the drillstring and gas release conditions were also determined. Results indicate that after total lost circulation, the drop in annular fluid level creates a suction effect, reducing the hydrostatic pressure of the fluid column in the water eyes of the upper MWD drillstring section. This leads to a low-pressure zone approaching to zero pump pressure, sometimes even negative pressure zone, inducing dissolved gases release and producing gas columns that block the transmission of MWD drilling fluid pulse signals. This is the fundamental reason why MWD signals cannot be detected at the surface. Measures such as increasing circulation rate and viscosity, re-injecting fluid into the annulus, reducing bit nozzle diameter, or replacing turbine tools are proposed to increase internal drillstring pressure, thereby reducing the likelihood of gas column formation and improving MWD signal transmission stability. These findings provide theoretical support and practical guidance for restoring MWD signals after total lost circulation in low-pressure formations drilling operation.