The invention relates to a rapid detection method for the decay of a liquid crystal display device having an LED backlight and a display device provided with a rapid compensating device for decay. The invention employs a mutually orthogonal series of driving signals to drive a plurality of LED devices in a synchronized manner with the driving signals having a one-to-one correspondence with the LED devices. A processing device extracts respective light emission data for the respective LED devices, compares the respective light emission data with the corresponding reference values pre-stored in the memory device and commands another device to compensate for any deviation existing therebetween. Accordingly, the LED devices are tested in batch mode and the testing is remarkably speeded up without interfering with users' activities.