A positioning device including a support tab, two pairs of sliding blocks slidably mounted in the support table, a positioning plate extending vertically from on a top of each sliding block, a base on which the support table is supported, a cylinder installed in a chamber of the base, a ball received in the cylinder, a piston rod slidably mounted in the cylinder that pushes the ball to move upward in a vertical direction, so that the ball drives the two pairs of sliding blocks to move away from each other in the first horizontal direction and the second horizontal direction, respectively, until the positioning plates of the two pairs of sliding blocks abut against four inner walls of the opening of the circuit board, respectively, to allow the electronic device to be received in a positioning slot defined by the positioning plates, and a release mechanism.