A positioning device working with a fixer for handheld, portable, mobile devices includes a securing member coupled to the fixer that serves to hold a mobile device in position. The securing member has a locking unit and a first connecting portion for engaging with the fixer. The securing member can secure the fixer onto an external article with the locking unit securing the securing member to the external article, so as to rapidly, removably suspend the mobile device at a nearby handrail. Thereby, the mobile device is strongly positioned over the external article by the locking unit and prevented from swinging or being displaced.