Herein, a polymeric thermo-optic (TO) mode switch based on an asymmetric directional coupler was fabricated. The device comprised a single-mode waveguide (SMW) and two-mode waveguide (TMW). The metal heater was deposited on TMW to achieve switching from the LP01 mode in SMW to LP11a mode in TMW by driving the voltage supplied to the heater. Using the traditional semiconductor microfabrication technology, the TO switch was fabricated on a Si substrate using EpoClad and SU-8 polymers as the cladding and core layers, respectively. When the operating wavelength was 1550 nm, the TO switch extinction ratio was 14.6 dB, driving power was 29.6 mW, and rise and fall times were 660.0 and 480.0 μs, respectively. The device has numerous potentials in reconfigurable mode-division-multiplexing systems.