Abstract: In order to study the fatigue failure mechanism of the driven gear component, a high-low cycle complex fatigue test was conducted on a turboprop engine propeller-shaft reducer component aiming for improving its working reliability and service life. The result shows that the fatigue crack initiated at the groove of the secondary driven gear test piece and propagated along the meridian plane of the shaft. To further analyze the mechanism of fatigue crack, a three-dimensional finite element model of the propeller-shaft assembly was established. The finite element software Ansys was used to calculate the stress of the secondary driven gear under the test conditions. The influence factors and law on the stress state of the groove has also been analyzed according to the calculation result. It is found that the magnitude of assembling interference and the size of fillets are the main reasons that affect the stress state of the groove. Finally, based on the result of the finite element analysis, suggestions and improvement measures are put forward, which provide an effective basis for failure analysis.