A residual toxicant detection device for testing residual toxicant in an aqueous solution is disclosed, which includes a first space cavity, a second space cavity, a connecting frame and a sensing cavity. The first space cavity includes a light source and a lens. The second space cavity includes a photo sensor and a circuit module. The connecting frame is configured to connect the first space cavity and the second space cavity. The sensing cavity for receiving an aqueous solution is formed between the first space cavity and the second space cavity and at one side of the connecting frame. The light source emits a light with a wavelength range. The photo sensor receives the sensing signal of the light passing through the sensing cavity. The circuit module is configured to calculate the absorbance and the variation in absorbance of the residual toxicants in the aqueous solution.