The radiation characteristics and the failure mechanism of MOS device were analyzed. The readout circuit of 320×256 radiation hardening ultraviolet(UV) focal plane array was designed by using Global Foundries 0.35μm CMOS process. The digital part of the circuit was hardened by using ring gate MOSFET and double ring protection. Furthermore, the SiO2-Si3N4 composite passivation layer was alternately grown on the chip surface readout circuit. Then the hardened readout circuit was tested by Gamma irradiation, and the output signal of the readout circuit was monitored by using an oscilloscope. By comparing the real-time irradiation state of the readout circuit before and after radiation hardening, it can be shown that the hardened readout circuit is greatly improved in radiation hardening, The total dose of anti-ionizing radiation was increased from 35krad (Si) to 50krad (Si).