A waveguide magic tee at W band is analyzed exactly and developed. The analysis approach is a combination of the mode expansion method and finite element method,whic h is based on the weak form of Helmholtz equation. Mode expansion instead of PML (Perfectly Matched Layers) is used to truncate the computing space of finite element. A magic tee at W band (75∼110 GHz) with the structure,whic h is easy for manufacture,is developed. Calculated results are presented and compared to those in experiments. Agreement has been observed. A sensitivity analysis is carried out, which shows the influence of the tolerance of manufacture on the performance of the magic tee. And the power carrying capacity of the designed magic tee is also evaluated.