The study area is situated in the central Sichuan hilly region. Due to the long-term deforestation of natural ever-green broad-leaf vegetation, this area had been suffered from soil erosion over time and was regarded as one of the most serious eroded region in the upper reach of the Yangtze River. In order to mitigate soil erosion, a large area of cypress (Cuppressus funebris)protection forests was established starting early 1970s. However, the current cypress forests were in a quite poor state in terms of stand structure, natural regeneration, stand stability, product supply and ecological functions. The objective of the study was to identify optimum stand structure aiming to improve soil conservation functions of the cypress monoculture. Through the survey of 48 sample plots and analysis of the relationship between canopy closure and the coverage and biomass of shrubs, grasses and litter, the canopy closures of 0.6-0.7 was regarded as the most suitable one, and 5 stand types of cypress monoculture was categorized, that is, low density type, medium-low density type, suitable density type, medium-high density type and over-dense type. The following results were found through a comparative study of the hydrological effects of the 5 stand types. (1) The maximal water-holding capacity of the litter in the 5 stand types depended on the total litter storage, and it ranked from higher to lower as: suitable density type (4.038 ± 0.497) t/hm²> medium-low density type (3.583 ± 0.521) t/hm²> medium-high density type (3.243 ± 0.455) t/hm²> low density type (2.841 ± 0.656) t/hm²> over-dense type (2.272 ± 0.580) t/hm². (2) The maximal water-holding capacity of soil was in the order of suitable density type (180.59 ± 14.83) mm > medium-low density type (173.84 ± 18.06) mm > medium-high density type(169.27 ± 13.20) mm > low density type (162.57 ± 8.79) mm > over-dense type (150.77 ± 5.08) mm; and the suitable density type had significant difference with other four types. (3) The data from runoff plot observation for six times of rainfall has indicated that the sediment yield of the 5 stand types ranked from lower to higher as suitable type (360.07 kg/ hm²) < medium-low density type (577.08 kg/ hm²) < medium-high density type (625.98 kg/ hm²) < low density type (878.51 kg/ hm²) < over-dense type (1026.74 kg/ hm²), and significant difference existed among the 5 stand types.