Abstract: When there is a foam column is the annular space of a pumping oil well there will be no distinct gas—oil interface. In that case, it is difficult to apply the acoustic well-sounding(AWS)method. Based on the principle of mass balance, this paper deals with rules of. motion of the pseudo-fluid level in the annular space in the process of testing the pumping oil well. Through working out the differential equation of motion of the pseudo-fluid level in annular space,P_wbo^KL(dL/dt)+m/k(e^KL-1)=(Q₁-Q₂TZP_at/AT_at) the depth of the pseudo-fluid level in annular space during the normal operation of the pumping oil well can be determined as following:.