The invention provides a catch basin with an incipient rain separation and purification function. The catch basin is characterized by comprising an inner well, an outer well, a well lid, a screen, a settling tank, a water return ring and biological stuffing; the inner well and the outer well adopt an inner pipeline structure and an outer pipeline structure which are different in height; the screen is connected to the top end of the wall of the inner well and the inner wall of the outer well; a cavity is formed among the inner well, the outer well and the screen; when incipient rain falls into the outer well, floatage and silt are blocked by the screen and the settling tank; rain overflows from the settling tank, passes the water return ring, uniformly passes through the biological stuffing, and is purified by microbes in the biological stuffing; purified rain water enters a coarse sand layer outside the catch basin through porous channels, and is then injected into groundwater; later rainfall falls into the inner well directly, and enters a rain water collection system through a water draining and collecting pipeline for discharging or recycling. The catch basin is simple in structure and maintenance management, small in occupied area, and obvious in economic benefit.