We investigate a tunable fluidic dye laser formed by a microcavity filled with a dye solution. We achieve a wide 18 nm tunability of the laser wavelength by controlling the cavity length for the first time. The microcavity is made of a silica capillary and two aligned fibers with end faces Au-coated. The Rhodamine 6G dye solution flowing through the microcavity is pumped by 532 nm wavelength laser pulses. Laser emission around 570 nm in the form of TE mode with a threshold of about 58 μJ/pulse is obtained. This work suggests a fiber-based convenient approach to achieve wavelength tunability and integration with lab-on-a-chip systems.