Abstract：The layer composite rockmass is soft and hard alternatly，and does not meet the conditions of homogeneity，continuity and isotropy. There are many problems to make the equivalent deformation moduli by field deformation test，among which the size-effect is a major problem. Combined with rockmass mechanics parameters selected for the layer composite rockmass of of Qingjiang-Shuibuya underground plant，numerical simulation experiments are made on the size-effect on deformation moduli of layer composite rockmass based on the field test of rockmass deformation. The results indicate that there are obvious size-effect for the studied layer composite rockmass，and the relation between deformation moduli and loading size can be represented by the negative power function，E = 7.989D－0.552. It is concluded that the loaded surface diameter should be up to 3～5 m，and equivalent rockmass deformation moduli are 3～4 GPa，about 70% lower than the results of general deformation test.
