The invention provides an inner-outer frame combined variable stiffness rock mechanical testing machine and method, and relates to the technical field of rock mechanics. The machine comprises a stiffness regulation device, a test loading device, a monitoring system and a control system. The stiffness regulation device comprises an outer frame top beam, an outer stand column, a stiffness regulationoil cylinder and a base, the loading oil cylinder of the test loading device loads and drives the inner top frame to rise so as to extend and deform the inner stand column and compress and deform a high-pressure spring; for the same test sample, in the set loaded load value condition, the loaded load of the stiffness regulation oil cylinder is increased, the displacement of the inner top beam andthe deformation of the inner stand column are reduced, and the loading stiffness is increased; when test is performed, according to the monitoring data of the displacement sensor, the control systemcontrols the load of the stiffness regulation oil cylinder to achieve the real-time control of displacement of the inner top beam so as to ensure that the loading stiffness in the whole test process meets a design value and is unchanged, solve the technical problem that the rock mechanical testing machine loading stiffness cannot be subjected to quantitative regulation and have the simple operation.