The waveguide ring resonator (WRR) is one of the key elements in the fields of integrated optical communications and optical sensors. The performance of these systems will be affected by the shape of the resonant curve. Aiming at the difference of transmission loss between the symmetric and anti-symmetric modes of the cavity coupler, and the reflection at the end face of the straight waveguide outside the resonator, the expression of the output line of the waveguide ring resonator is obtained using the principle of multi-beam interference superposition. The high aspect ratio silicon nitride WRR is tested with an optical vector network analyzer based on single-side band modulation. The results confirm the effect of the coupler mode and the reflection at waveguide end face on the resonance curve. The actual test curves can be well fitted with the theoretical simulation results.