Taking small watershed as a unit, acxeording to the allocating feature of mountainous protection forest system,and adopting locating and semi-locating research technique, the function of water accumulation and soil conservation of mountainous protection forest system was monitor,d and analyzed in hierarchical way. The research results are as follows. The difference of soil and water conserative effectis relatively large in different land types, different land use methods and different forest types. The main source of sand producing is the bare area of central Guizhou plateau and cultivated slopeland in drainage area, The erosion quantity of cropland is 18.7-36.0 times than that of forest land; The erosion quantity of bare land is far larger than that of cultivated slopeland, and is as high as about 100 times than that of forest land. After the protection forest system was established, the protection effect was obvious, the mlulus of soil erosion reduced following the increase of forest cover rate The modulus of soil erosion decreased 36% when the forest cover rate of drainage area increased from 18.6% to 42.4%.After the construction of plantation in sloping field, the surfac, mnoff decreased from 79.4% to 96.9%，and the silt amount d,creas,d from 87.8% to 99.9%;In order to control soil and w ester loss of small westershed in mountainous region, forest cover rate must be more than 30%.