A collapsible filtration tank for a dust collector includes an outer frame, an engaging base and a filter. The outer frame is formed cylindrical, having plural meshes formed in its circumferential wall. The engaging base is formed annular, installed around the opening of the outer frame. The filter is inserted in a chamber of the outer frame and restrictively blocked by the blocking edge of the engaging base. Thus, the filter can be independently replaced with a new one instead of shifting the whole filtration tank.