The existing optical switches and splitters usually have only a single function and are often polarization-sensitive. Based on waveguide structure, optofluidics and multimode interference coupling theory, here a composite device that combines the optical switch and splitter is proposed. By driving the index-matching liquid to enter or exit two multimode cavities and the self-imaging effect of multimode interference, the optical path switching or splitting is achieved. The structure is optimized and the switch and splitter performances are discussed by simulation. The research results show that the insertion loss and crosstalk of the optical switch are extremely low for 1550 nm wavelength. At the two output ports, the insertion losses are 0.034 dB and 0.026 dB, and the crosstalks are -33.26 dB and -50.9 dB. When the device is used as a 1 × 2 splitter, the splitting ratio is uniform(1:1 for 1550 nm wavelength), and the total insertion loss is as low as 0.026 dB. The proposed device has the advantages of simple structure, easy operation, multifunctionality, polarization independence and flexible application. It can be applied in multi-path optoelectronic systems, optical communications and other fields.