Benefit from the elastic element in the drivetrain, the variable stiffness actuators offer advantages over traditional rigid actuators in security, robustness, energy consumption. However, it is also very bulky due to the integration of the elastic element, especially with large deflection and high load. This article introduces a novel planar high-compliance joint, in which the torsion spring is not an independent part but a synergy result of two adjacent parts developed from the stiffness adjustment linkage mechanism. The flexible element is a cantilever leaf feature belongs to the rigid-flexible integral linkage. The deflection and load ability of the torsion spring can be enhanced by arranging multiple uniform stress leaf branches on the integral linkage without increasing the size. Explicit analytical formulas are derived in a form convenient for leaf parameters design and verified by the finite element simulation and prototype experiments. Finally, the error factors are quantitatively analyzed, which can provide information for the leaf correction design. This kind of planar high-compliance joint is expected to be an ideal choice for the variable stiffness actuators that require stringent axial compactness with large deflection and high load.