The quasirectangular tunnel has many advantages such as high utilization of space and onetime shaping of doubleline. The mechanical properties of such tunnels are different from that of traditional ones. The shield tail is the weakest part of the shield, hence it is necessary to study its mechanical properties. A numerical simulation model of quasirectangular shield tail used in Ningbo Rail Transit Line 4 is established; and then the mechanical properties of the ideal structure model and model considering initial geometric defect under different working conditions are analyzed; finally, the influences of shell thickness and soil thickness on deformation distribution of the shield tail are studied by parameterization method. The conclusions are drawn as follows: (1) The design of shield tail meets the strength and rigidity requirements of construction. (2) The initial geometric defects greatly affect the deformation of shield tail. (3) The thrust friction affects the stress and deformation little; and the steering load mainly increases the outward deformation. 