A CMOS compatible PC/Nwell/Psub double junction photodiode pixel was proposed, which can efficiently detect fluorescence from CsI(Tl) scintillation in an X-ray sensor. Photoelectric and spectral responses of PC/Nwell, Nwell/Psub and PC/Nwell/Psub photodiodes were analyzed and modeled. Simulation results show PC/Nwell/Psub photodiode has larger photocurrent than PC/Nwell photodiode and Nwell/Psub photodiode, and its spectral response is more in accordance with CsI(Tl) fluorescence spectrum. Improved PC/Nwell/Psub photodiode detecting CsI(Tl) fluorescence was designed in CSMC 0.5 m CMOS process, CTIA(capacitive transimpedance amplifier)architecture was used to readout photocurrent signal. CMOS X-ray sensor IC prototype contains 8×8 pixel array and pixel pitch is 100×100 m2. Testing results show the dark current of the improved PC/Nwell/Psub photodiode(6.5 pA) is less than that of PC/Nwell and PC/Nwell/Psub photodiodes(13 pA and 11 pA respectively). The sensitivity of PC/Nwell/Psub photodiode is about 20 pA/lux under white LED. The spectrum response of PC/Nwell/Psub photodiode ranges from 400 nm to 800 nm with a peak at 532 nm, which is in accordance with the fluorescence spectrum of CsI(Tl) in an indirect X-ray sensor. Preliminary testing results show the sensitivity of X-ray sensor IC under Cu target X-ray is about 0.21 Vm2/W or 5097 e-/pixel @ 8.05 keV considering the pixel size, integration time and average energy of X-ray photons. A CMOS compatible PC / Nwell / Psub double junction photodiode pixel was proposed, which can efficiently detect fluorescence from CsI (Tl) scintillation in an X-ray sensor. Photoelectric and spectral responses of PC / Nwell, Nwell / Psub and PC / Nwell / Psub photodiodes were analyzed and modeled. Simulation results show PC / Nwell / Psub photodiode has larger photocurrent than PC / Nwell photodiode and Nwell / Psub photodiode, and its spectral response is more in accordance with CsI (Tl) fluorescence spectrum. / Psub photodiode detection CsI (Tl) fluorescence was designed in CSMC 0.5 m CMOS process, CTIA (capacitive transimpedance amplifier) ​​architecture was used to readout photocurrent signal. CMOS X-ray sensor IC prototype contains 8 × 8 pixel array and pixel pitch is 100 × 100 m2. Testing results show the dark current of the improved PC / Nwell / Psub photodiode (6.5 pA) is less than that of PC / Nwell and PC / Nwell / Psub photodiodes (13 pA and 11 pA respectively). The sensitivity of PC / Nwell / Psub phot The spectrum response of PC / Nwell / Psub photodiode ranges from 400 nm to 800 nm with a peak at 532 nm, which is in accordance with the fluorescence spectrum of CsI (Tl) in an Preliminary testing results show the sensitivity of X-ray sensor IC under Cu target X-ray is about 0.21 Vm2 / W or 5097 e- / pixel @ 8.05 keV considering the pixel size, integration time and average energy of X-ray photons.