Variable stiffness joints are a kind of compliant mechanisms which can improve the safety of physical human-robot interaction which has attracted much attention in recent years. Elastic elements are used in the internal kinematic structures of variable stiffness joints. In this paper, Elastomers with magneto-rheological fluids (MRFs) properties have great potential application for variable stiffness joints (VSJs) and implantable bioelectronics devices by allowing actuators to rapidly and reversibly changed from a “fluid” to a “solid-like” state and precisely controlled within a very short time and over a wide range under a magnetic field. A new adjustable stiffness composites (ASCs) that combine an outside liner (PDMS) with a chamber (MRFs) is introduced. When improving the magnetic field intensity, the MRFs hardens and the stiffness increases by as much as 4–5 orders of magnitude. In order to solve the bi-material nested cantilever beam subjected to a concentrated tangential force at the free end of the beam, a theoretical approach is proposed by means of the Airy stress function method together with the stress function test solution. By comparing the results obtained from the theoretical and experimental measurements, a very good agreement is found, showing the accuracy and feasibility of the theoretical results. This solution will be useful in analyzing cantilever beam with arbitrary variations of bi-material nested and it can serve as a basis for establishing stiffness and stress theories. Due to the highly deformable of the ASCs, it has great potentially for our future soft robot design and manufacture work.