During the construction of a tunnel bored by a double shield tunnel boring machine (TBM), many long cracks were found on the inner surface of the side segments. The positions, quantities and widths of the cracks are summarized after field investigations, and the causes for the cracks are analyzed in terms of TBM operation, rock mass conditions, ground water conditions, segment structures and backfilling grouting conditions. It is concluded that the pulling of the thrust cylinders and the constraint of the steel shell of the shield tail are the main causes for the cracks on the inner surface of the segments. Finally, countermeasures to minimize the cracks are proposed.