Abstract: The present invention is a prewet system for a filter. The present invention includes a power controller. A first container is used for storing iso-propyl alcohol liquid. An air pump is connected to the first container via a liquid pipe. The air pump is utilized for sucking the iso-propyl alcohol from the first container and driving the iso-propyl alcohol liquid to flow in the prewet system. An impurity filter is connected to the air pump via a liquid pipe, which is useful for filtering out impurities of the iso-propyl alcohol liquid. A second container is connected to the impurity filter for holding a filter. Between the filter and the second container is a pressure detector for detecting the liquid pressure inside the pipe. The prewet system provides a better prewet effect than the conventional one.