In practice, asymmetric formation disturbance occurs due to the sequence of excavation though parallel double-track tunnel is a typical symmetrical engineering. Based on a shield tunneling project of a section of the Suzhou Rail Transit under the subgrade of the Shanghai–Nanjing Expressway, a finite element model was established to obtain a numerical solution that was validated by the measured data to guarantee reliability. According to the construction characteristics of the shield method, two correction coefficients—namely the soil loss rate correction coefficient α and the settlement trough width correction coefficient β—were introduced. A modified Peck formula suitable for the preceding tunnel and the subsequent tunnel was proposed. The applicability of the modified Peck formula was verified by another similar project. The results showed that the numerical solution can better reflect the actual settlement of the highway subgrade under shield tunneling. The results calculated by the classic Peck formula had a large error in comparison with the measured data. The modified empirical formula could more accurately predict the settlement of the expressway subgrade caused by the shield method when α1 = 0.38 and β1 = 2.08 for the preceding tunnel and α2 = 0.29 and β2 = 1.99 for the subsequent tunnel.