The construction process of underground engineering is a stress process of structure in timespace. The stress process of first primary support, second primary support and temporary support of extralarge span tunnel of enlarging section of Xinkaotang Tunnel Portal during construction is analyzed. The analytical results show that: 1) The support stress state shows the procedural dependency; and the final state is not always the worse. 2) The second primary support is effective; and the risk of support disassembling can be brought under control. 3) The safety factor of second primary support is the largest, followed by that of first primary support, and that of temporary support is the smallest.