A portable extension memory storage device comprises a body and a slide positioning member. The body includes an elastic positioning tongue. The slide positioning member includes a positioning rib to be engaged into a positioning concave in the positioning tongue. With the elastic positioning tongue, the abrasion can be reduced, and the service life can be extended. The positioning concave offers an assured and firm engagement to improve the convenience in use. The assembly of the portable extension memory storage device can be finished without using any other threaded elements or locking elements, thus reducing the material and labor cost.