The absolute measurement of thermal X-ray flux from a hohlraum is essential to get the radiation temperature and the laser-hohlraum coupling efficiency in the experiments of indirect-driven inertial confinement fusion (ICF), which can be measured by a flat response X-ray detector. The configuration parameters of flat-response compound filters were optimized to enhance the performance of flat-response X-ray detectors. The effects of thin layer thickness, thick layer thickness and hole area ratio on response flatness of detectors were investigated. The calculated results agree well with the calibrated data. From the results, it can be found that the thin layer thickness plays a primary role in performance of detectors while thick layer thickness and hole area ration take a minor role. Besides, the detectors show a best response flatness less than 5% which reaches the limit value of theory as the thin layer thickness, thick layer thickness and hole area ratio are 50 nm, 380 nm and 1:6.1.