A CMOS compatible P+/Nwell/Psub double junction photodiode pixel was proposed, which can effi- ciently detect fluorescence from CsI(T1) scintillation in an X-ray sensor. Photoelectric and spectral responses of P+/Nwell, NweE1/Psub and P+/Nwell/Psub photodiodes were analyzed and modeled. Simulation results show P+/Nweu/Psub photodiode has larger photocurrent than P+/Nwetl photodiode and Nweu/Psub photodiode, and its spectral response is more in accordance with CsI(T1) fluorescence spectrum. Improved P+/Nweu/Psub photodiode detecting CsI(T1) 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 μm2. Testing results show the dark current of the improved P+/Nwell/Psub photodiode (6.5 pA) is less than that of P+/Nwell and P+/Nwell/Psub photodiodes (13 pA and 11 pA respectively). The sen- sitivity of P+/Nwell/Psub photodiode is about 20 pA/lux under white LED. The spectrum response of P+/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 Csl(T1) 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 V.m^2/W or 5097e-/pixel @ 8.05 keV considering the pixel size, integration time and average energy of X-ray photons.