Total ground settlement caused by adjacent level parallel twin shield tunnel construction which conforms to the normal distribution curve is put forward. Based on the stochastic medium theory of single tunnel, aiming at the existing deficiencies, a modified simplified method of stochastic medium theory is established to calculate its ground settlement caused by adjacent level parallel twin shield tunnel construction; and it is able to consider the settlement curve asymmetry at the same time. As is shown in analytical calculation, the proposed method is relatively consistent with the measured values; and the calculation is relatively simple. The calculation parameters are studied; and measured data statistical results show that the radial constringency of twin shield tunnel is greater than the first tunnel; but the influential angle becomes smaller due to impacts of the first tunnel construction. Formula (6) is proposed to define the distance range and the total ground settlement curve shape is connected to the value of P. The definition of distance range is when P less than or equal to 1.1.