Abstract: The lateral enlarging excavation section is simplified to double connected domains by use of the method of equivalent circle. Combined with the theory of complex function, the specific equation of complex stress function for deformed ultra-large section (span 29.171 m and depth-span ratio 0.45) is deduced by Schwarz’s alternating method during lateral enlarging excavation. Taking the Henglongshan tunnel in Shenzhen of China as an example, the stress and displacement characteristics of surrounding rock mass are analyzed by numerical calculation. It is shown that the width of 4 m is preferable for each enlarging excavation, and during three steps of enlarging, the stress states of rock after second, third, and fourth excavation are different. The maximum value of stress and displacement is presented in the approaching region of double holes, and the average stresses of tension and compression around the main hole are larger than those around the enlarging holes. The allowable difference of stress at top of arch is controlled within 4%, and the computed results are precise.