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Medium-power gyrotron operating in high frequency usually operates in harmonics to reduce the required guiding magnetic field strength. Low guiding magnetic field is in favor of compact gyrotron, which makes it easier to apply the gyrotron to large scale devices and to extend the applications of gyrotron to wider fields. However, high harmonic modes suffer from low intensity of cyclotron radiation and additional competition with neighboring fundamental modes. As we all know, single mode operation can be realized by using a special startup scenario and a positive magnetic field taper can increase output efficiency. The influence of magnetic field taper on gyrotron operation is studied by generalizing the well-known η⊥μ,F plot, which just discussed the transverse efficiency using Gaussian field profile and not took the self-consistent non-stationary codes or ohmic dissipation into consideration. In this paper, linear and self-consistent nonlinear codes are used to investigate influences of magnetic field taper on start current and output parameters of a 394 GHz gyrotron, which is designed for enhancement of Nuclear Magnetic Resonance spectroscopy through Dynamic Nuclear Polarization. Simulation results show that when the magnetic field taper is +0.04 T the maximum output power is obtained and the maximum value is 3% larger than that corresponding to uniform magnetic field. Appropriate startup scenario is designed for single mode operation and multimode simulation is carried out to demonstrate that mode TE261+ dose dominate in mode competition in the designed startup path.