All fluorescent white organic light-emitting diodes (WOLEDs) have aroused significant interest owing to the advantages of excellent operational stability, low efficiency roll-off and low cost. However, it is challenging to further improve the efficiency through the design of the device structure. Here, we demonstrated all fluorescent WOLEDs with high efficiency and low efficiency roll-off using a phosphor sensitization strategy. It can be seen that the energies of triplet excitons can be efficiently transferred to singlet excitons owing to charge carrier trapping and energy transfer processes between the host, phosphor sensitizer and fluorescent emitter. The resulting phosphor-sensitized fluorescent WOLEDs showed maximum current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) values of 42.4 cd A⁻¹, 36.4 lm W⁻¹ and 14.3%, and these values were retained at 34.0 cd A⁻¹, 19.8 lm W⁻¹ and 11.6% at 1000 cd m⁻² luminance, respectively. Furthermore, the resulting WOLEDs also exhibited stable electroluminescence (EL) spectra. The Commission Internationale de l’Eclairage (CIE) values showed very little variation from (0.41, 0.47) to (0.39, 0.46) over a large luminance range from 500 to 5000 cd m⁻². This demonstrates that using a phosphor to sensitize the fluorescent emitter is a potential method for developing high efficiency, low efficiency roll-off fluorescent WOLEDs with stable EL spectra.