A method for manufacturing a natural pesticide and a water quenching device thereof are provided. The method includes the steps of: burning paddy at a high temperature of 400-900° C. into carbonized paddy, wherein the carbonized paddy comprises at least 50% silica, at least 10% organic matter, at least 2% potassium, and at most 2% water； using a water quenching device to quench and heat the carbonized paddy, enabling the carbonized paddy to form a gas and a solid substance by means of solid-liquid separation； using the water quenching device to condense the gas separated from the carbonized paddy into a liquid； and testing a PH value of the liquid after condensation through the water quenching device, wherein when the PH value of the liquid is alkaline, the liquid is outputted and stored as a pesticide.
