A positioning device that has a support table, two pairs of sliding blocks slidably mounted in the support table, a positioning plate extending vertically at the top of each sliding block, a base on which the support table is supported, a cylinder in the base, a ball in the cylinder, a piston rod slidably mounted in the cylinder and pushing 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 that drives the piston rod to move downward in the vertical direction to allow the two pairs of sliding blocks to be moved toward each other in the first horizontal direction and the second horizontal direction, so that the positioning plates drive the electronic device in the positioning slot to move toward a center of the opening.