In the process of Hydraulic fracture in fractured formation，the interaction between hydraulic fracture and natural fractures made the natural fracture open or shear slip，which leads to the fluid loss in fractured reservoirs dramatically，at the same time，the hydraulic fracturing risk increased greatly.However，at present most of the leak-off models aims at homogeneous formation only.On the basis of natural fracture dynamic loss process，this paper builds a natural fracture leak-off physical model.Based on the combined pressure continuation and volume conservation，we proposed the leak-off mathematical model which is originated from microscopic understanding of the fluid leak-off mechanization of the fractured formation.We programed in the light of the idea and method shown above in this paper.The calculated results of model shows fracturing fluid，the natural fractures and construction parameters can affect fluid loss.Through calculating and analyzing the affecting factors，the conclusion is that the model can fit to practical project，and it has a strong engineering significance.