Provided are a test device and a test method for the hydraulics size of shear flow polymer. The device comprises a liquid barrel, a shear flow generator, a filter liquid collection container and a support for supporting the liquid barrel and the shear flow generator, wherein the liquid barrel is provided with a piston matched with the inner diameter of the liquid barrel, the lower end of the liquid barrel is communicated with the shear flow generator, the shear flow generator is composed of an upper chuck, a lower chuck, an upper layer filter membrane and a lower layer filter membrane, the upper layer filter membrane and the lower layer filter membrane are arranged on the lower chuck, a liquid outlet pipe is positioned at the lower portion of the lower chuck, a control valve is positioned on the liquid outlet pipe, and the filter liquid collection container is positioned under the liquid outlet pipe. The method using the test device comprises the steps of preparing polymer solution, filtering and drawing relation curve between viscosity retention rate and corresponding hole diameter of the lower layer filter membrane. When the viscosity retention is 70%, the hole radius of the corresponding lower layer filter membrane is the average hydraulics radius of the shearing rate.