The invention provides a simulated cutting force loading device for a three-axis boring mill and a use method. The device comprises a rigid framework, an X-direction position adjustment assembly, a Y-direction cutting force loading assembly, an X-direction position adjustment assembly and a cutter rotation-direction cutting force loading assembly. The method comprises steps as follows: the simulated cutting force loading device is hoisted on a bench of a machine tool, the position of a simulating facing head is adjusted through the X-direction position adjustment assembly and the Y-direction position adjustment assembly and fixed, and a boring bar on the simulating facing head and a boring bar on the facing head of the machine tool are fixedly connected; a group of cutting force is set, pretightening bolts in the Y-direction cutting force loading assembly and the cutter rotation-direction cutting force loading assembly are tightened through a torque wrench, so that friction force in the cutting force loading assemblies is equal to the set cutting force, and friction force needs to be read out after the tightening torque is calculated according to the formula M=KFd/mu due to the fact that the friction force cannot be directly read; the machine tool is started and runs machining programs, the actual operation state of the machine tool is simulated really under the force bearing condition, and the performance of the machine tool under the current cutting force condition is tested.