The present invention relates to a downhole device for being moved downwards in a well by fluid to assist stimulation of a production zone of the well, the well comprising a well tubular structure having a first opening and a first movable sleeve arranged opposite the first opening, and the well tubular structure having an inner diameter, the downhole device having an axial extension, and comprising: a first part comprising two projection elements having a profile matching grooves in the sleeve, and a second part comprising: a body, two anchor elements projectable from the body for anchoring the second part in the well tubular structure, and a sealing element configured to seal against the well tubular structure, the downhole device further comprising: a displacement mechanism comprising a piston movable within a piston cylinder to displace, in the axial extension, the first part in relation to the second part when anchored in the well tubular structure to operate the sleeve. The invention also relates to a downhole system for stimulating a formation surrounding a well tubular structure of a well. Finally, the present invention relates to a stimulation method for stimulating a formation by means of a downhole system according to the invention.