Abstract: The studies on the leakage characteristics of submarine oil pipelines mostly focus on the leakage at the sea surface or the large-aperture underwater vertical leakage. And the leakage characteristics of crude oil through micro-apertures in the horizontal direction are seldom analyzed. In this paper, therefore, horizontal leakage experiments were conducted to figure out the relationship between the flowing time of crude oil to the water surface and the leakage under different pipeline internal pressures and leakage apertures. Then, according to the multiphase flow and numerical calculation theories, a numerical model for the horizontal leakage of underwater pipelines was established to simulate the effects of parametric change (e.g. sea current, oil density and gas volume fraction) on the movement forms of horizontal leakage of underwater pipelines. It is indicated that the higher the sea current velocity, the longer the rising time of the oil to the water surface and the greater the distance of horizontal drift. For a constant leakage aperture, the higher the pipeline internal pressure, the greater the leakage and the longer the movement time of crude oil in the horizontal direction. The research results provide the theoretical basis for the treatment of pipeline leakage. (8 Figures, 3 Tables, 20 References)