Abstract: Differential pressure compensating pump is a new one that can generate underbalance low pulse,it is also called underbalance pulse deep oil well pump.This paper presents the lab test results and theoretical analysis of the pump.Based on the test data,this paper highlights the whole process of the forming of pump’s cavity and the working fluid level fluctuation when the formed cavity is opened instantaneously.Lab test results indicate that at the same simulation condition,the fluid-withdraw rate of the differential pressure compensating deep oil well pump is far high than that of the conventional deep oil well pump.The main stimulation basis is as follows:during down stroke of the differential pressure compensating pump,the cavity is formed and opened instantaneously,then,Bernoulli effect and underbalance pulse effect are generated,more oil are produced.