In order to ensure the safe operation of fermentation tank, the safety assessment of fermentation tank with pit defect in axial arrangement was evaluated in accordance with GB/T19624-2004 "Safety Assessment of Pressure Vessels with Defect in Use". At the same time, the static analysis of multi-pits with different numbers and distances was carried out by ANSYS. The results show that when the axial arrangement multi-pits under the condition that the ratio of defect depth to thickness is 2/3, that is the pit defect depth is 4 mm, the tank is safe even though there are pits with different number and distances that the design pressure is always less than the maximum allowable working pressure. Moreover, there is a certain regulation about the maximum stress and stress concentration factor between the number of pit and the distance of adjacent pit. The maximum stress value and stress concentration factor are directly proportional to the number of pit defect, while inversely proportional to the distance between adjacent pit. Especially, the value variation range is larger when the distance L/X≤3, but tends to be constant after L/X > 3.