The twin tubes of a shieldbored tunnel cross underneath the operating No.1 line of Guangzhou Metro successfully, with the minimum vertical distance between the twin tubes and the operating Metro tunnel being 2.342m. In the paper, the geological conditions, the features and difficulty of the project and the mechanisms of the ground surface settlement and building settlement caused by shield boring are presented. The measures taking during the boring of the tunnel under the condition of no preconsolidation of No.1 Metro line, which includes full inspection and repairing of the shield equipment, accurate control of the thrust force, pressure and muck discharging rate during boring, timely simultaneous grouting and secondary grouting and timely monitoring measurement, are expatiated. For the sake of these measures, the safety of No.1 Metro line is guaranteed.