A tunable polymer waveguide coupler based on Mach-Zehnder interferometer (MZI) with thermal isolation trenches (TITs) is proposed and analyzed. The temperature distribution in the cross section of polymer waveguides with and without TITs is simulated, which illustrates that the TITs can reduce the power consumption efficiently. The proper thickness of up-cladding waveguide is chosen through investigating the propagation loss of guided modes under aluminium electrode. According to the power consumption influenced by the distance between the TIT and the metal electrode, the optical loss induced by the mode mismatch of the waveguides with and without TITs, and precision of the fabrication craft, the position of TITs is determined. To realize the splitting ratio from 1 to 0, the power of the designed tunable polymer waveguide coupler with TITs is 2.20 mW, which is only 47.4% of the coupler′s power without TITs.