Downhole casing bending and external pressure collapse combined loading test tool and test method

The invention discloses an underground casing bending and external pressure collapse combined loading test tool and a test method. The tool comprises a bending and external pressure loading unit and a control unit, the loading unit comprises a kettle body, a bending loading device, an external pressure loading device, a strain sensing device and a pressure sensing device; the test method comprises the following steps: pasting the strain sensing device on the test sleeve; a to-be-tested sleeve penetrates through the kettle body, and the kettle body is filled with a liquid medium; setting the bent dog-leg degree of the to-be-tested sleeve; the first bending loading device and the second bending loading device perform bending loading on the radial direction of the to-be-tested sleeve, the strain sensing device feeds back, and when the bending dog leg degree reaches a set value, the bending loading is stopped; the bending loading condition is kept unchanged, the liquid medium in the kettle body is pressurized unt