An ultrasonic plasticizing capillary rheometer comprises a frame, a rheological test unit, an ultrasonic vibration apparatus, and a tool head moving and loading unit. The ultrasonic vibration apparatus is provided with an ultrasonic tool head capable of extending into a plasticizing chamber along an axial direction, providing extrusion force for polymer in the plasticizing chamber, and providing ultrasonic wave energy required by the polymer for fusion from a solid state to a liquid state. In the ultrasonic plasticizing capillary rheometer, three steps including heating fusion, ultrasonic vibration for fluidity increase, and material extrusion are completed through the ultrasonic tool head and in the plasticizing chamber, thereby simplifying the conveying path. A heating ring providing the required initial temperature for the plasticizing process sleeves a material canister, provides a constant material canister temperature for the rheometer, and reduces the test error. A filter screen is arranged in the material canister, centralizes fused polymer, and improves the test accuracy. During a test process, a viscosity test method is accurate in heating, short in fusion time, and wide in application range, is slightly affected by the external environment, and can provide viscosity data high in accuracy.