The design and preparation of 10GHz~50GHz ultra-wideband micro coaxial filter based on rectangular micro coaxial fabrication technology is studied under the requirement of RF microsystems for high-performance ultra-wideband filters. The model of micro coaxial filter is based on micro coaxial transmission line circuit, and its simulation results show that the insertion loss is less than 0.8dB, the return loss is superior to 17dB, and the out-of-band suppression is up to 45dB. This 10-layer-filter is completed through rectangular micro coaxial fabrication process after nine lithography-electroformingflattening processes and a support structure lithography. The test results of the filter samples show that the insertion loss is 0.6dB and the return loss is 21.6dB at 40GHz, the restraint outside the band are 41.8dB and 48.9dB at 5GHz and 55GHz, the curve of which is consistent to the simulation curve. The ultra-wideband rectangular micro coaxial filter reflect the advantages of rectangular coaxial structure well，such as low less and high isolation.