Abstract: Charge sharing phenomenon due to load capacitance's memory effect in switched-capacitor sample-and-hold(S/H) circuit is modeled,which gives it may cause the opamp settling amplitude increasing.According to the total equation derived from the settling behavior of opamp,the increased amplitude may further cause more power,larger bandwidth or lower speed.For improving this phenomenon,an optimizing methodology is proposed based on pre-discharge theory and double sampling technology,which can eliminate the extra requirement of power and bandwidth without reducing the initial speed of S/H circuit.A 12bit,100Msps double S/H circuit achieved by simulation proves the availability of this method.