The invention relates to a direct-acting magneto-rheological fluid shock absorber with controllable stiffness, which belongs to the technical field of vibration control. The shock absorber includes the controlled object and the magnetorheological fluid shock absorber, wherein the controlled object is the vibration control object, and the magnetorheological fluid shock absorber is composed of a support plate, a slide rail, a slider, a connecting plate, a lock nut, Gasket, piston rod, upper piston, outer sleeve, excitation coil, inner sleeve, damping channel, magnetorheological fluid, lower piston, floating piston, nitrogen chamber and working cylinder. The stiffness of the magnetorheological fluid in the magnetorheological fluid damper can be controlled by changing the current of the excitation coil, so as to meet the internal resonance requirements between the controlled object and the magnetorheological fluid damper in the system. The damping consumes vibration energy to achieve the purpose of vibration reduction. The invention utilizes the internal resonance phenomenon for vibration reduction, has obvious vibration reduction effect, strong robustness and less energy consumption, and has good practical value in the vibration reduction of vehicle engines, flexible mechanical arms and other fields.