This paper takes a helical gear shaft as research subject and discusses the main structural parameters to see how it would affect natural frequency sensitivity. The parametrical model of helical gear shaft is created by using software PROE3.0. The natural frequencies and vibration shapes are calculated by the modal analysis. The relationship between the structural parameters and the natural frequency sensitivity can be obtained by means of ANSYS. The result shows that the structural parameters can affect the natural frequency sensitivity. The variation in low order natural frequency is similar to that in high order natural frequency. The effect of module on the natural frequency sensitivity is more remarkable comparing with pressure angle, modification coefficient and helical angle. Selecting structural parameters reasonably may reduce vibration and noise of the helical gear shaft effectively.