The invention provides a device for collecting and utilizing rainwater to irrigate farmlands. The device comprises a machine body, wherein a rack is arranged at the lower end of the machine body; a water collection tank is arranged at the upper end of the left side of the rack; the lower end of the water collection tank is connected with a water purification tank through a water pipe; the water purification tank is arranged at the lower end of the left side of the rack; the right side of the water purification tank is connected with a water storage tank through a water pipe; the water storage tank is arranged on the right side of the rack; an irrigation device is arranged on the right side of the top of the water storage tank; trundles are arranged at the bottom end of the rack; a water inlet pipe is arranged on the left side of the water collection tank; a rainwater collection device is arranged at the top of the water collection tank; a first control box is arranged on the right side of the water purification tank; a top cover is arranged at the top of the water storage tank; a second control box is arranged in the middle of the top of the top cover; the top of the second control box is connected with a solar heat collection plate through a bracket. The device is used for sufficiently collecting the rainwater for irrigating, so that water resources can be effectively saved; the device is powered by solar energy and has the advantages of high efficiency and energy saving.