Evanescent wave fiber-optic sensors （EWFS） with acicular encapsulation were fabricated using silicon photolithography technology and silica wet-etching technology. This type of EWFS is small in size, consumes little reagent, suffers less from deformation during the detection process and also protects the sensors from pollution. The results obtained from the EWFS with different sensing fiber lengths were studied. The effect of the fiber length on the sensors absorbance were analyzed theoretically and experimentally. It was shown that the properties of the EWFS could be improved greatly by choosing suitable length of the sensing fiber, and the contribution of unit length of sensing fiber to the absorbance becomes less as the sensors total length becomes longer.