Portal crane with span convenient to adjust

The invention discloses a portal crane with convenient span adjustment, which comprises a main beam, a left support leg, a right support leg and a cart travelling mechanism, the left support leg and the right support leg have the same structure and respectively comprise two branch legs, and the bottom ends of the two corresponding branch legs are fixed on the top surfaces of the front part and the rear part of a main transverse beam of the same cart travelling mechanism; an electric hoist is mounted at the lower part of the main beam; the two corresponding branch legs are located on the front portion and the rear portion of the left end or the right end of the main beam, lower protruding supporting portions are arranged on the lower portions of the opposite positions of the upper portions of the two branch legs, and supporting extending portions horizontally extending outwards are arranged on the front portion and the rear portion of the upper portion of the main beam. The left supporting leg and the right su