Hydrajet fracturing is a novel stimulation technique developed in recent years, with mechanism that inner static pressure boosts by transforming flow kinetic energy into the static pressure energy resulted from the continuous flow velocity decrement after being injected into the perforation, based on the Bernoulli principle. The formation of the innerouter perforation pressure difference is the key point for the successful field application and it is important to find the relationship between the inner pressure field and outer ambient pressure as well as the influence factors. Based on the continuous medium theory and considering the high speed jet, the jet flow control equations are put forward and through the numerical simulation of inner flow field with standard k-ε twoequation model, analysis on influence factors is further conduct in this paper. It is suggested from the research results that the jetting pressure is positively correlated with boost pressure while jetting distance is negative. The higher the jetting pressure is, the more favorable for hydrajet fracturing, and the longer the jetting distance is, the more unfavorable. The outer ambient pressure is uncorrelated to the boost pressure with little influence.