statistical optimum design method suitable for temperature-insensitive waveguide coupler is proposed. Using the method, 120 nm waveband fluorinated polyimide waveguide 3 dB coupler is designed with the optimization of the temperature fluctuation. The design result is verified by the 3D-BPM simulation, and shows that the coupling ratio of (0.500±0.007) in the waveband of 1490～1610 nm and temperature region of -10～40 ℃ is realized. The measuring method for refractive index dependence on temperature and for dispersion characteristic of the fluorinated polyimide film is also reported