System for assembling/dissembling blades of windmills, designed to provide a number of devices and means that allow to carry out the operations of ascent/descent of a blade of windmills for the purposes of assembling/dissembling of said blade in/from a rotating hub of the windmill, the system including: • a main structure, capable of moving along the whole extension of a tower of a windmill up to the top of such tower and including supporting means to hold a blade that are fastened to the rotating hub of the windmill; • means of adjustment to the geometry of the several models of blade, incorporated in pincers of said main structure, which adjusting means consist of turning high arms and low arms that when in operation, force supporting elements to keep a constant pressure with the surface of the blade thus assuring a minimum static friction, higher than the weight of the blade, while the main structure moves along the length of said tower during the ascending/descending phases; • a hoisting structure associated to tower, which constitutes the element for supporting the load during operations of ascent/descent of a blade of the windmill, characterized in that the hoisting structure has sufficient capacity to transmit the stress directly to the tower without abutting less resistant parts as the windmill or the rotating hub, through supports that generate pressure and sufficient capacity to modify its own geometry and avoid possible collisions with the tower during the ascent/descent of the mentioned hoisting structure by means of hydraulic mechanisms, further characterized in that it includes: • a set of movable lug nuts that allow a controlled movement in the horizontal level of the main structure to tightly control its position, and • means for lifting the main structure which include four traction cables, two on each side, that cooperate with said hoisting structure and that at one end are tied to a respective side of said movable lug nuts of that main structure and at the other end are fastened to the shaft of a respective capstan of that same side. • a main structure, capable of moving along the whole extension of a tower of a windmill up to the top of such tower and including supporting means to hold a blade that are fastened to the rotating hub of the windmill; • means of adjustment to the geometry of the several models of blade, incorporated in pincers of said main structure, which adjusting means consist of turning high arms and low arms that when in operation, force supporting elements to keep a constant pressure with the surface of the blade thus assuring a minimum static friction, higher than the weight of the blade, while the main structure moves along the length of said tower during the ascending/descending phases; • a hoisting structure associated to tower, which constitutes the element for supporting the load during operations of ascent/descent of a blade of the windmill, characterized in that the hoisting structure has sufficient capacity to transmit the stress directly to the tower without abutting less resistant parts as the windmill or the rotating hub, through supports that generate pressure and sufficient capacity to modify its own geometry and avoid possible collisions with the tower during the ascent/descent of the mentioned hoisting structure by means of hydraulic mechanisms, further characterized in that it includes: • a set of movable lug nuts that allow a controlled movement in the horizontal level of the main structure to tightly control its position, and • means for lifting the main structure which include four traction cables, two on each side, that cooperate with said hoisting structure and that at one end are tied to a respective side of said movable lug nuts of that main structure and at the other end are fastened to the shaft of a respective capstan of that same side.