We presented a pneumatically tunable 2 x 2 optofluidic switch for on-chip light routing that was controlled by compressed air. The device was fabricated with an optically clear elastomer-polydimethylsiloxane ( PDMS)-by soft-lithography. The optical switching is realized with a tunable air-gap mirror by which the light is deflected due to total internal reflection in the bypass state. When the device is subjected to high pressure, the air gap collapses and hence the light will be switched to the crossover state. The device had a switching speed of more than 5 Hz and an extinction ratio of 8 dB. This switch can be readily integrated with other microfluidic circuits. We demonstrated a simple reconfigurable optical waveguide circuit for dual-channel microfluidic spectroscopy measurement on a chip.