In order to resolve the gain mismatch issue for the Time-Interleaved ADC in wideband radar RF sampling, a gain estimation method based on least square curve fitting is proposed. Based on the gain fluctuation of ADC, the negative influence of gain mismatch on the output signal spectrum was analyzed. According to the characters of wideband DAR echo, the gain estimation curve was obtained. The concrete implementation steps of this estimation method were also presented. Experimental results prove the validity of the proposed method in the wideband TIADC system.