In this study. We designed a Buck on-chip low-dropout regulator(LDO)circuit that can reduce the overall circuit losses. It primarily analyzes the losses of the on-chip power rail generation circuit LDO in the Buck converter. It automatically switches the power supply rail of the LDO from the input voltage to its output voltage by reusing the output of the Buck converter when a relatively high output voltage of the Buck converter is detected. Thus， the losses on the power transistor of the LDO can be reduced and the conversion efficiency of the Buck converter can be improved. This LDO circuit can be used in Buck converters with a wide input range. The experimental results indicate that under the typical working conditions with an input voltage of 12 V and an output voltage of 5 V，the LDO based on this structure can improve the conversion efficiency of the Buck converter by up to 5%.