In this paper, the effects of two optimal underwater antiwashout admixtures which were polypropylenetype PVAD and cellulosetype HPMC on the fresh concrete properties, fluidity and selfcompacting degree of concrete made with the technique of single addition and mixed addition respectively were studied. And the effects of the addition of them on the antiwashout resistance and strength of underwater concrete which was cast by tremie method were studied. The results indicate that the antiwashout resistane and strength of underwater concrete prepared with the technique of mixed addition is better than those of underwater concrete with the technique of single addition.