Abstract: To analyze the size effect of hydraulic fracturing, the multi-scale single hole model for hydraulic fracturing is established considering the stress characteristics of steeply dipping surrounding rock, the extended forms of acoustic emission signal, crack and pore pressure evolution in the process of hydraulic fracturing are analyzed, the trends of fracturing pressure for different scale models are summed up. The results show that although each model has different sizes, the development characteristic of crack is similar, which mainly develops along the vertical direction of drilling hole center, and has a shape of triangular at the direction of top and bottom in model; the existence of size effect makes the failure pressure increase with the larger of specimen. Water pressure shows a good correlation with R/r, the demand of water pressure is bigger with the increasing range of damage.