In a mistake-proof monitoring method applied to a bedridden care system, the bedridden care system includes a computer, a plurality of transmitters and a warning device. The computer will generate a shutdown message to notice a nurse to check whether the transmitter is shut down maliciously, if no sensing message is transmitted from any transmitter within a predetermined time or a predetermined number of times. The computer will generate a detachment message to the warning device, if the transmitter is contacted improperly with a sensor. The computer will generate an error message to the warning device, if the time interval between the current time and a previous time occurring an abnormal situation is beyond a predetermined normal time interval and the sensor is not installed at a correct detected position. The mistake-proof monitoring method can immediately discover whether or not the sensors or the transmitters are operated at normal conditions.