Abstract: Objective This paper investigates the influence of Ni element on the low-temperature toughness value A_\mathrmKV_2 (−40 ℃) of deposited metal of C-Mn-Ni-Cu all-position flux-cored wire for crude oil tanker corrosion-resistant steel, aiming to determine the optimal Ni value. Methods Seven types of all-position flux-cored wires with varying Ni contents were designed and developed for crude oil tanker corrosion-resistant steel. Deposited metals with Ni contents ranging from 0% to 1.16% were prepared using 100% CO₂ gas shielded welding. Optical microscopy (OM) and scanning electron microscopy (SEM) were employed to study the effect of Ni content on the microstructure evolution of the deposited metal of all-position flux-cored wire for crude oil tanker corrosion-resistant steel. Mechanical properties were evaluated through impact tests. Results The results show that with the Ni content increasing from 0% to 0.29%, the microstructure of the weld zone transitions from proeutectoid ferrite (PF)+massive ferrite(MF)+ acicular ferrite (AF) to AF. Little martensite (M) structures appear in the incomplete normalizing zone, and the low-temperature toughness value A_\mathrmKV_2 (−40 ℃) increases from 54 J to 139 J. When the Ni content rises from 0.29% to 1.16%, the weld zone microstructure shifts to ultra-low-carbon bainite (ULCB), accompanied by an increase in M structures in the incomplete normalizing zone. Consequently, the A_\mathrmKV_2 (−40 ℃) value decreases from 109 J to 71 J. Conclusion The optimal low-temperature impact toughness value A_\mathrmKV_2 (−40 ℃) for the deposited metal of C-Mn-Ni-Cu all-position flux-cored wire for crude oil tanker corrosion-resistant steel is achieved when the Ni content is 0.29%.