Optical waveguide has a promising application as absorption cell in concentration measurement of gases and liquids. The sensitivity of the measuring system is directly proportional to the effective path length (EPL) of the absorption cell. A mathematical model is proposed for the EPL of various kinds of waveguides. Theoretical analysis is made for the dependence of EPL on the waveguide parameters such as inner-coated film thickness and film refractive index, bore-diameter and bending radius, and intensity distribution of incident light. Several important results are obtained from the simulations, which provide optimizing parameters for the sensing system and algorithms for error compensation. Calculation results have shown good agreement with published measured data.