In order to investigate the effects of new twin tunneling on the longitudinal displacement of existing shield tunnel, Pasternak double parameters of the elastic foundation and Euler -Bernoulli beam model is employed to consider the new tunnel, existing tunnel skew angle and the tunnel excavation causing factors such as ground loss. The new double line tunnel excavation caused by free field vertical displacement of surrounding soils is determined by Loganathan-Poulos solution and superposition principle, and null space method is used to calculate the settlement deformation of the existing tunnel. Taking Shenzhen Metro Line 9 passing under Metro Line 4 as an example, the predicted results by the null space method is compared with the field-measured data. The effect of horizontal distance, ground loss, skew angle and depth of the new tunnel on existing tunnel settlement is further analyzed. The results show that the distance between the two lines of the new tunnel significantly affects the settlement of the existing tunnel in a range of 7 times its outer diameter. The settlement decreases with the increase of the distance, the settlement domain increases as the distance increases. The settlement pattern changes from "V" to "W". When the distance is larger than 7 times the outer diameter of the new tunnel, the settlement and pattern tend to be stable. In addition, ground loss, skew angle and the depth of the new tunnel significantly influences the settlement. It increases with the increase of ground loss, and decreases with the skew angle and the depth of the new tunnel.