In this paper, the causes and mechanisms of the upfloating of the segment rings of a shieldbored tunnel on No. 7 Line of Guangzhou Metro, which is located in fullface hardrock ground, are analyzed in terms of mechanical conditions and space conditions, so as to ensure the quality of the segment lining. Countermeasures, including pea gravel backfilling behind the segment rings, prevention of gout loss by means of grout stopping rings and waterstopping rings, improving the backfilling effect of the simultaneous grouting by adopting semiopen boring mode, and installing cylinders at the bottom of the front shield to avoid shield pitching, are proposed.