A thermo-optical tunable filter of flat-top response is successfully prepared characteristic of flat-top response is obtianed by dividing half-wave resonant cavity into two parts having different thickness through wet corrosion method,and making the area of the two parts irradiated by incident light substantially equal.The output response of the filter is measured,and all the experiment results are consistent with the theoretical analyses.The difference of the maximum relative transmittance and the minimum one is less than 0.01.Compared with other methods of obtaining flat-top response,the method is advantageous in its simpler manufacturing process,more optimal of performance,and facility to integrate with other devices.Moreover,in this paper,the mechanism of obtaining narrow-band thermo-optical tunable filter of flat-top response by using a resonant cavity of scores of micrometers is proposed.The fluctuation of that filter is less than 0.02,and the bandwidth is less than 1 nm.