For the shield construction of the subway tunnel, the “upper soft and lower hard” stratum layer is the most difficult to drill through. It is characterized by the fact that the drill bit is easily subject to damage and drill bit replacement cannot be carried out under atmospheric pressure or being pressurized. According to the particularity of the construction in Tianhai District, Shenzhen City, we adopt the method of stabilizing the cutting ring of the shield machine and the front part of the cutter head by rotary shotcreting pile and of mortar backfilling in the bin , which has completely solved the difficulty in drill bit replacement under “upper soft and lower hard” geological conditions, ensuring the safety of the person for drill bit replacement and making reboring with low cost and within short time.