Abstract: Disclosed are nucleated red blood cell warning devices and methods, and flow cytometers using the same. The devices, methods and flow cytometers can warn whether nucleated red blood cells exist in a blood sample. The warning device may obtain forward-scattered light information, side-scattered light information and fluorescence information when cells in the blood sample pass through a detection region of the flow cytometer. The warning device may generate a side-scattered light-fluorescence dot plot, so as to classify leucocytes into four groups, and may generate a forward-scattered light-fluorescence dot plot, where the forward-scattered light-fluorescence dot plot can include a leucocyte group region. The warning device may obtain a predetermined feature region located at the left side of the leucocyte group region, perform statistics on the amount of characterization cells in the predetermined feature region, and provide a warning when the number of the characterization cells exceeds a threshold value.