Shiziyang tunnel on GuangzhouShenzhenHong Kong passengerdedicated railway, which has a design speed of 350km/h, is a milestone in underwater tunnel construction in China. Properly understanding the technical difficulties of the works, timely carrying out relative scientific researches and taking corresponding measures have great importance to ensure the quality of the tunnel. The paper can provide reference for the construction of similar superlong highpressured underwater tunnels in the future.