Abstract: The borehole wall stability is closely related to drilling fluid flow pattern in sedimentary strata, this paper combined a 2386m salt drilling project in Yewu sag basin, conducted the experiment in different depth with different pump discharge and return velocity. The result shows:in water sensitive formation (clay, mudstone, sandy shale etc.) with shallow rock porosity and high plasticity index, in the drilling fluid laminar flow conditions, the maximum diameter shrinkage rate was 26.6%, in middle and deep formation, in the turbulent flow conditions, borehole shrinkage and oversized exit exist in same time. With the depth increased, diameter shrinkage rate tends to decrease; the ratio of oversized diameter increases with borehole depth. In mudstone formation, the maximum diameter oversized rate is up to 157.4%, in salt rock formation, the maximum diameter oversized rate is 225.7%. This paper aiming at these problems conducts the theoretical analysis and research, and gives the detail suggestions.