An automatic sensing and warning device for placing an intravenous drip includes a transmitter unit, a blocking unit, a receiver unit, and a fixing device. The transmitter unit is attachable to an injection site on a patient's hand and is operable to transmit an identification signal. The receiver unit is arranged on the blocking unit and includes a drive circuit and a warning device. The fixing device supports the blocking unit and the receiver unit on the intravenous drip pole. The RF receiver is set at a predetermined vertical distance from a drip chamber and areas above and below the RF receiver are respectively defined as dangerous and safe areas. When the injection site is spaced from the drip chamber by a distance shorter than the predetermined distance, the RF receiver receives the identification signal from the RF transmitter and the drive circuit activates the warning device to issue warnings.