A mobility assistive device includes a first frame including a first support and a first wheel, a second frame including a second support and second wheels, and a connecting structure including first connecting boards connected to the first support. The second support includes a supporting rod and abutting portions. Each first connecting board has a slot for being engaged with the supporting rod, an abutting surface for abutting against the abutting portions, and a perforation. Each positioning member is disposed on one the second support, and includes a positioning portion. When the slots are engaged with the supporting rod and the abutting surfaces are abutted against the abutting portions, the positioning portions are movable between a locked position and a released position. When in the locked position, the positioning portions are inserted into the perforations. When in the released position, the positioning portions are drew out from the perforations.