Abstract: China Spallation Neutron Source(CSNS) beam power will upgrade to 500 kW(CSNS-II), energy gain of H^- linac will up to 300 MeV from 80 MeV, and the 648 MHz β_g=0.6 5-cell superconducting elliptical cavity will be adopted to accelerate the H^- from 150 to 300 MeV. Accompanied with the advantage of high accelerating gradient, simple structure, easy post-processing, weak stiffeness and detuning is the shortcoming of elliptical cavity, this study analysis detuning characteristic of the elliptical cavity. With code COMSOL Multiphysics, helium pressure sensitivity coefficient K_P and lorentz force detuning factor K_L of bare cavity was calculated under two end fixed conditions, K_P=–45.705 Hz/mbar(1 mbar=100 Pa) and K_L=1.574 Hz/(MV/m)². Stiffener ring was designed and optimized to reduce the Helium pressure sensitivity coefficient and lorentz force detuning factor, and double ring was chosen to improve the detuning of elliptical cavity at last by vast calculation and analysis, the position of stiffener ring was optimized at 75 and 120 mm under two end fixed boundary, Helium pressure sensitivity coefficient drop down to K_P=6 Hz/mbar, lorentz force detuning factor drop down to K_L=0.43 Hz/(MV/m)². Moreover, elastic boundary was used to analyze helium pressure sensitivity coefficient and lorentz force detuning factor, more realistic results were got under tuner end 30 kN/mm boundary and the other end fixed condition, K_P=4.8 Hz/mbar and K_L=1.99 Hz/(MV/m)², meet engineering requirements. In addition, dynamic lorentz force detuning was analyzed qualitatively with code CST. And natural frequency of cavity was calculated by code Workbench, results show that nature frequency is far from RF pulse repetition rate and ambient vibration frequency, no resonance happen.