A method and system for drying an active part of an electric transformer, by continuously determining a moisture content in the solid insulations during vapor phase drying in a drying oven and stopping this drying when an equilibrium moisture content is reached. Then, continuously determining a moisture content in the solid insulations during the retightening and geometric adjustment phase of the active part outside the drying oven and continuing with the determination of moisture content during vacuum drying once the active part is inside the hermetically sealed tank, and stopping vacuum drying when an equilibrium moisture content is reached in the solid insulations. The determination of moisture content and equilibrium moisture content is carried out by applying the diffusion equation.