Abstract: During shield tunnelling gaps will emerge between segment rings and surrounding rock, which will affect rock stability and structural safety of the segment. In this paper, considering the geological characteristics and synchronous grouting requirements of the Najing Weisan Road river-crossing tunnel, a test was carried out regarding one-component inert grouting material to study the working capacity and compression strength of grouts with different cement/sand ratios. The test results show that the grout working capacity meets the requirements of synchronous grouting if its cement/sand ratio is over 0.55. Considering strength requirements and economical factors, the one-component inert grout with a mix proportion of group D4 was ultimately adopted. The monitored surface settlements were not beyond the alarm range during shield tunnelling, which proves that the synchronous grouting achieved the expected results.