Abstract: A double-grid ionization chamber(GIC) based on the 10B(n,α)7Li and 6Li(n,α)T nuclear reaction has been developed for neutron energy spectrum measurement in the energy region below 150 keV of CSNS/Back-n by time of flight method. According to the strong adjoint gamma-flash features, the structure design of thin window and thin bottom lining is adopted. In view of the characteristics of strong gamma-flash neutron beam, the structure design of thin bottom lining is adopted. Garfield++, SRIM and Simcenter Magnet Electric simulation programs are used to simulate the working parameters such as working gas, electrode spacing and electric field distribution of the GIC. CF₄, which has fast electron drift speed and small diffusion coefficient and great organizing ability, is selected as the working gas for the CSNS/Back-n beam test with cathode-grid and grid-anode spacing of 20 mm and 5 mm, respectively. The collecting area within 74 mm can be considered as the uniform electric field area, and the relative deviation of the field strength is not more than 0.03%. The performance of the GIC was tested by α source. The GIC has good energy resolution for ²³⁹Pu/²⁴¹Am/²⁴⁴Cm mixed α surface source with CF4. And the best energy resolution is 2.4%@5.48 MeV. Compared with the measured results of the parallel-plate chamber and the silicon microstrip detector, the advantages of energy resolution of the GIC developed in this work are verified.