A CMOS-compatible optical biosensing system based on visible absorption spectroscopy is proposed. Within the compact prototype, the CMOS sensor consisting a P+/Nwell finger photodiode and a transimpedance amplifier (TIA) is implemented in a standard 0.35-mum CMOS technology. The ABTS/H2O2/HRP method is adopted as the ground for medical diagnosis. Experimental results demonstrate that the minimum concentration successfully detected for H2O2 is 1muM. The detection limit is one order of magnitude better than a traditional CMOS biosensor, and comparable to a commercial spectrophotometer.