The accurate measurement of dynamic liquid depth is a key technology to analyze the production and operation dynamics of oilfields, and to formulate and adjust oilfield development plans. Because of the conventional pontoon method and acoustic wave reflection method, which cannot be applied to inclined wells due to the influence of the casing annular space, the dynamometer calculation method has poor stability due to the influence of continuous alternating load, and a large amount of calculation is required in the dynamic liquid level prediction method. Starting from the dual-source characteristics of electric energy as both a power source and an information source, a soft measurement method is proposed to correlate the cross-domain correlation between the working electrical parameters of the above-ground motor, the suspension load of the pumping unit, and the dynamic surface depth of the underground oil well. Firstly, according to the energy flow and transfer mechanism of the beam pumping unit during the pumping process, a mathematical model of the donkey head suspension load is formulated by the input electrical parameters of the ground motor, the parameters of the four-link mechanism, and the inclination displacement of the beam. Secondly, based on the fact that the pressure drop of the oil flow in the fixed valve of the pump presents zero value at the moment of opening of the fixed valve of the upstroke and the process of the downstroke, the area between the peak point of the first wave and the trough point of the first wave is dynamically intercepted as the optimal. Finally, according to the correlation between the depth of the dynamic liquid level and the static load on and below the suspension point of the pumping unit, the mathematical model of soft measurement of dynamic liquid level is established, and the input electrical parameters of the collected motor are substituted into the calculation to obtain the soft measurement value of dynamic liquid level depth. The results of engineering experiments and applications show that the method has good stability and strong engineering practicability. It can be applied to electric drive wells. The relative measurement error is not more than ±8%.