Provided is a residual toxicant detection system and a residual toxicant detection method, the residual toxicant detection method including: allowing an aqueous solution containing a residual toxicant to flow into a detection portion including a cavity; providing at a side of the cavity a light containing a specific wavelength range to react with the residual toxicant; receiving the light that passes through the cavity on another side of the cavity, thereby generating a sensing signal; and calculating an amount of change in absorbance of the aqueous solution according to the sensing signal, wherein when the amount of change in absorbance is less than a threshold value, a detection count is accumulated, and when the accumulated detection count is greater than a predetermined value, a detection result is generated. Therefore, whether or not the residual toxicant on the object is cleaned can be determined easily and accurately.