The utility model relates to a portable coaxiality measuring instrument which comprises a measuring bench, a mandrel arranged on the central area of the measuring bench, and positioning columns symmetrically arranged at both sides of the mandrel. The positioning columns and the mandrel are connected through a double-crank mechanism in the measuring bench, and the positioning columns move synchronously in the measuring bench through the double-crank mechanism. An adjusting and positioning mechanism is arranged at one end portion of the measuring bench, and the adjusting and positioning mechanism is connected with corresponding positioning columns in the measuring bench after penetrating through the measuring bench. The adjusting and positioning mechanism drives the positioning columns to move in the measuring bench and positions the positioning columns after movement, and the mandrel is provided with a pivoted lever of which one end is connected with the mandrel and the other end is connected to one end of an indicating gauge mount. The pivoted lever is rotatable about the mandrel, and the other end of the indicating gauge mount is provided with an indicating gauge for measuring coaxiality. The portable coaxiality measuring instrument is compact in structure, convenient for use, and wide in application scope. The portable coaxiality measuring instrument is safe and reliable and can rapidly measure the coaxiality of rotary parts vary in dimension.