The invention provides a combined flower box with rain collecting and dirt filtering functions and relates to the technical field of municipal engineering. An upper cover with water-permeable holes is arranged on the upper portion of a water tank, a flower box body is arranged on the upper cover, flower box water-permeable holes are formed in the bottom of the flower box body, a plant soil layer, a ceramsite pervious concrete layer and a dirt filtering layer are distributed in the flower box body from top to bottom, water absorbing ropes are arranged in the flower box water-permeable holes and the water-permeable holes in the upper cover in a penetrating mode, and the upper ends of the water absorbing ropes are disposed in the ceramsite pervious concrete layer; rain collecting awnings are arranged on the outer side of the flower box body, and water guide pipes are arranged between the rain collecting awnings and the water tank. By means of the combined flower box, rainwater can be supplied to flowers and grass for growth for a long time, and manual flower watering frequency is reduced; the ceramsite pervious concrete layer and the dirt filtering layer are arranged in the flower box body, so pollutants in rainwater can be effectively filtered away, and environment protection is benefited; the rain collecting awnings also have a sun-shading function, and the outer side of the flower box body at the upper end of the water tank can be used for people to sit and rest.