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In the efficient way of high-frequency reciprocating action, the testing machine of shaft bending fatigue is widely used to test the fatigue limit and life of many kinds of shaft and beam parts in mechanical engineering. An accuracy exciting frequency should be generated by the shaker to cause the shaft parts bending resonance in the testing machine. Thus an accuracy analytical determination method of the bending resonant frequency is necessary for the design and application of the complex testing machine system. In this paper, the testing machine system is simplified and modeled as an 8-dofs nonlinear coupling dynamic system, and its governing equation is derived. The nonlinear dynamic system is simulated and analyzed in the view of steady state response and its dynamic stability, and the dominant factors on the bending resonance are identified in the dynamic system. With a linearized reduction and eigenvalue approximation, an accuracy analytical frequency formula at the shaft bending resonance is determined. Error analysis of the simplified resonant frequency formula is carried out. Moreover, the resonant frequency formula is verified by applying finite element analysis to the testing machine.