This study systematically investigates the influence of key neutron beam parameters—specifically flux and energy spectrum—from the China Spallation Neutron Source - Atmospheric Neutron Irradiation Spectrometer (CSNS-ANIS) on Single Event Burnout (SEB) testing for silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs). The experimental results reveal two critical findings: First, under a constant neutron energy spectrum, varying the incident flux (from 8.7×10⁵ to 7.3×10⁶ n/cm²/s for E_n > 10 MeV) does not significantly alter the SEB failure rate, confirming the linearity assumption for flux scaling in accelerated tests. Second, and more importantly, modifying the beam spectrum via flux controller (tungsten blocks) induces significant spectral “hardening”. This hardening, characterized by an increased proportion of high-energy neutrons, leads to a measurable increase in the calculated failure-in-time (FIT) rate for the same total fluence, demonstrating a spectrum-dependent effect. To elucidate the underlying physics, a combined simulation methodology was employed. The Geant4 toolkit was used to model neutron transport and secondary ion generation within the ANIS beamline and device geometry, reproducing the measured spectral shifts. Subsequently, TCAD device simulations were performed to analyze the charge deposition and carrier transport dynamics triggered by the secondary ions, linking the varied ion Linear Energy Transfer (LET) spectra under different beam conditions to the observed SEB cross-sections. The analysis confirms that at a high drain-to-source voltage (V_DS = 1200 V), the SEB threshold LET is sufficiently low, making the failure rate less sensitive to spectral shape as most secondary ions exceed the threshold, explaining the convergence of FIT values under different spectra at this bias. This work not only validates the performance and flexibility of the ANIS facility for controlled radiation effects testing but also provides crucial empirical insights for future ANIS users, highlighting the necessity of reporting and accounting for the detailed beam spectrum—not just the total flux—in neutron single-event effect tests to ensure accurate and comparable reliability assessments.