A positioning device for positioning an object includes a platform, a slide rail vertically extending from the platform, two standard units, and a retaining unit. The standard units are slidably mounted to the slide rail to abut against a first side of the object to cooperatively define a position reference plane for the object. The retaining unit includes a fixing block fixed to the high standard unit, a slide bar slidably mounted to the fixing block, a holding arm fixed to the slide bar to abut against a second side opposite to the first side of the object, and a spring to bias the slide bar to ensure that the holding arm abuts against the second side of the object.