<p>In shield tunnel projects, the shield machine has to stop and open the slurry chamber under pressure to replace the cutterhead tools, clear the obstacles ahead or repair the pipeline gates. During this period, the airtightness capacity requirement of filter cake is much higher than tunnelling period. The polymer-modified slurry composed of hydrolytic polyacrylamide (HPAM), natrium carbonicum, aluminum sulfate and Na-bentonite was developed for chamber opening under pressure period. A high-pressure infiltration test device was assembled to simulate the pressure-maintaining process of the filter cake. Both the limit airtightness pressure and the limit airtightness duration in the stratum with different grain composition was tested. The test results show that the unique “bridging network” and “adsorption” characteristics of the HPAM molecule can reduce the additive amount of bentonite by 40% to 85%, shorten the formation time of filter cake by 20% to 50%, and increase the limit airtightness pressure by 10% to 40%. In high permeability sandy pebble stratum, conventional bentonite slurry cannot form filter cake, while the limit airtightness duration of the filter cake formed by HPAM slurry reached to 1096 s and the airtightness duration under normal air pressure surpassed the minimum requirement in practice engineering by 115%. The research results could provide references for the optimization pathway of slurry preparation during chamber opening under pressure.</p>