An optical fiber sensing system for biomedical and biochemical sensing was developed. The system was designed for multiparameter sensing to pH, SO2, NH3/NH4+. In the developed system, two light emitting diodes are used as light sources, a silicon photo-diode is used as light signal detector, a multimold stepped bifurcate optical fiber bundle with great aperture is used as light transmissive medium, the dyestuff is coupled to fiber by different methods. The new design idea is double beam compensation. The system is small, cheap but stable and sensitive, its response for pH is about 0.01 pH, the detective limits are 2 micrometers ol/L for NH+4 in water, 2 X 10-4 mol/L for SO32- in water, respectively. The system is powered with battery, so can be used for fieldwork.