Abstract: To effectively solve the line position problem caused by the inconsistency between existing metro engineering reserved conditions and current engineering conditions when the external factors has changed, a new method of connecting the metro extension line—symmetry method is proposed. At the same time, the cyclotomic method commonly used in railway design is applied to metro line design, and its characteristics are compared with the symmetry method when the front transition curve is gentle and the line position of some circular curves is unchanged. Research results show that cyclotomic method can also be applied to the design of metro lines, and can better solve the problem of inconsistent line position between construction reserved conditions and current situation. Under the condition that the length of circular curve is determined and the length of front and back transition curves is consistent, symmetry method can be applied to the design of metro lines and simplify the process of line adjustment.