In order to meet the design requirement of front-end readout circuit for radiation detectors, a fully on-chip fast transient response LDO was designed in a 0.18 μm standard CMOS process. A large output swing high gain amplifier was utilized to drive the pass transistor, which could increase the gate voltage swing of the pass transistor. Thus, the dimension of the pass transistor and the dropout voltage were reduced. The loop-gain of LDO and the charge/discharge gate current of the pass transistor were increased, which is helpful for improving the transient response. In order to keep LDO stable under all load current conditions without sacrificing the loop-gain bandwidth and the die area, a frequency compensation method based on load current partition was proposed. The simulation results demonstrate that the phase margin is greater than 53° at the load capacitance of 200 nF and the load current ranging from 0 to 200 mA. With the help of large output swing and high gain amplifier, the maximum output current can be increased by two times when the dimension of pass transistor is the same. The settling time is smaller than 6.5 μs when the load current suddenly increases from 10 mA to 200 mA. The die area of the proposed LDO is 120 μm×264 μm. The power efficiency at full load is 97.76% and the minimum dropout voltage is 50 mV.