The invention discloses a high-isolation microwave test fixture for ultrahigh-power GaN microwave devices, which belongs to the technical field of testing of semiconductor devices. The high-isolation microwave test fixture comprises a test box body, a PCB (printed circuit board), a holddown, a direct-current bias leading-out terminal and a radio frequency input/output leading-out terminal, wherein the PCB is arranged in the text box body, and the PCB comprises a feed matching circuit and a radio frequency input/output microstrip; the feed matching circuit is divided into two parts, i.e. a grid feed matching circuit and a drain feed matching circuit, the grid feed matching circuit comprises double sector-shaped wires, two RC (Resistor-Capacitor) filter networks, a grid bias resistor and a grid filter capacitor, and the drain feed matching circuit comprises double sector-shaped wires, two RC filter networks and a drain filter capacitor; the direct-current bias leading-out terminal is led out of the feed matching circuit from two feedthru capacitors; the radio frequency input/output leading-out terminal is a coaxial transmission-to-microstrip transmission connector. The invention solves the problem of insufficient isolation of leaked radio frequency signals of a drain electrode existing in the process of ultrahigh-power testing of microwave devices, can prevent a direct-current bias module from being burnt, and ensures the authenticity and effectiveness of test results.