Combining with a large-diameter bored pile example,this article analyzes the main influential factors of the R.C.concrete cover in cast-in-situ pile,i.e.steel bar spacing and coarse aggregate size,slurry density and thickness of mud cake,concrete slump and the pile hole diameter,concrete impulse and tremie pipe depth,thickness of concrete cover design and construction errors,and puts forward prevention measures,to provide reference for similar bored pile engineering.