Integrator is a kind of circuit, which is used to accumulate the input signal and calculate the result. It usually uses some resistors, capacitors and operational amplifiers. In this paper, the basic concept of the 1st-order integrator is first introduced, then the theoretical analysis, software simulation and practical experiment are carried out. Finally, how to build, design, test and characterize a 1st-order integrator is understood. It is also pointed out that in application, a 1st-order integrator circuit is typically used to eliminate the integral compensation in control according to feedback.