A method for assembling a mixer-container intended to receive a biopharmaceutical fluid in order to mix it, wherein: - a container is provided, having a flexible wall defining an inner space suitable for filling with biopharmaceutical fluid, the container comprising: o a mixing device having a shaft having an adjustable length along a main axis, the shaft extending rectilinearly between a lower end and an upper end, o a first bearing attached to the wall, the shaft extending at least into the inner space from the first bearing, - a rigid outer containment device of the container is provided, - a drive motor is provided, located outside the container, the motor being suitable for rotating the shaft of the mixing device, - the container is placed in the rigid outer containment device, the rigid outer containment device comprising a bottom wall and a peripheral wall defining a housing suitable for receiving the container, the flexible wall of the container being arranged on the bottom wall of the rigid outer containment device, and - knowing that the position of the motor is fixed relative to the rigid outer containment device and that the container is already placed in the rigid outer containment device, the length of the shaft along the main axis is adjusted by arranging the shaft facing the motor, on the side of the upper end, in order to enable the motor to rotate the shaft, the shaft extending vertically along the main axis with the lower end located towards a lower part of the container while the upper end is located towards an upper part of the container, being connected to the first bearing, and wherein the shaft has a first part and a second part which are movable in translation relative to one another along the main axis, the first part comprising an element suitable for sliding in a rectilinear slot of the second part of the shaft by adjusting the length of the shaft, and wherein the first bearing is placed in the immediate vicinity of the motor, at a distance from the motor less than the length (If) of the rectilinear slot. - a container is provided, having a flexible wall defining an inner space suitable for filling with biopharmaceutical fluid, the container comprising: o a mixing device having a shaft having an adjustable length along a main axis, the shaft extending rectilinearly between a lower end and an upper end, o a first bearing attached to the wall, the shaft extending at least into the inner space from the first bearing, o a mixing device having a shaft having an adjustable length along a main axis, the shaft extending rectilinearly between a lower end and an upper end, o a first bearing attached to the wall, the shaft extending at least into the inner space from the first bearing, - a rigid outer containment device of the container is provided, - a drive motor is provided, located outside the container, the motor being suitable for rotating the shaft of the mixing device, - the container is placed in the rigid outer containment device, the rigid outer containment device comprising a bottom wall and a peripheral wall defining a housing suitable for receiving the container, the flexible wall of the container being arranged on the bottom wall of the rigid outer containment device, and - knowing that the position of the motor is fixed relative to the rigid outer containment device and that the container is already placed in the rigid outer containment device, the length of the shaft along the main axis is adjusted by arranging the shaft facing the motor, on the side of the upper end, in order to enable the motor to rotate the shaft, the shaft extending vertically along the main axis with the lower end located towards a lower part of the container while the upper end is located towards an upper part of the container, being connected to the first bearing,