Abstract: This research selects two common ancient trees(ancient Pinus tabulaeformis and ancient Platycladus orientalis),and two common turfgrasses(Poa pratensis and Ophiopogon japonecus) in Beijing,in order to find the influence of the grass to the ancient trees underlaying surface,we measured microbial numbers of the soil in depth 0-40cm.The number of the bacteria,fungi and actinomycete in 0-20cm layer is higher than 20-40cm,the number of the bacteria is higher than fungi and actinomycete．The two grasses increase the number of soil microorganisms of the old trees in depth 0-40cm,and the Poa pratensis is better than the Ophiopogon japonecus.The two grasses can all increase denitrifying bacteria in 0-40cm and reduce the number of nitrogen-fixing bacteria in the layer of 0-40cm and the two of this trend is not significant.The two grasses can all increase Cellulose-decomposing bacterium’s number in 0-40cm layer．