A composite ultraviolet (UV)/blue photodetector structure has been proposed, which is composed of P-type silicon substrate, Pwell, Nwell and N-channel metaloxide-semiconductor field-effect transistor (NMOSFET) realized in the Pwell. In this photodetector, lateral ringshaped Pwell-Nwell junction was used to separate the photogenerated carriers, and non-equilibrium excess hole was injected to the Pwell bulk for changing the bulk potential and shifting the NMOSFET’s threshold voltage as well as the output drain current. By technology computer-aided design (TCAD) device, simulation and analysis of this proposed photodetector were carried out. Simulation results show that the combined photodetector has enhanced responsivity to UV/blue spectrum. Moreover, it exhibits very high sensitivity to weak and especially ultral-weak optical light. A sensitivity of 7000 A/W was obtained when an incident optical power of 0.01 μW was illuminated to the photodetector, which is 35000 times higher than the responsivity of a conventional silicon-based UV photodiode (usually is about 0.2 A/W). As a result, this proposed combined photodetector has great potential values for UV applications.