Oil-water two-phase flow in a complex pipeline, consisting of varying pipes and fittings in series/parallel, is commonly encountered in the petroleum industry. However, previous study has been mainly limited to a single constant-radius pipe. In this paper, a unified model of oil-water two-phase flow in a complex pipeline is developed based on the combination of pipe serial-parallel theory, flow pattern transformation criterion, two-fluid and homogenous models. A case is presented to verify the unified model and compare with CFD results. The results show that the unified model performs well in predicting both the flow distribution and pressure drops of oil-water two-phase flow in a complex pipeline. Compared with CFD results for water volumetric fractions ranging from 0% to 100%, the highest absolute percentage errors of the proposed model is 14.4% and the average is 9.8%.