If product oil pipelines were constructed in the areas of complex topography, parts of pipelines would inevitably encounter some problems via the large gap areas. Due to the existence of pipeline crossing point and the physical difference of the transport oil, it may appear the phenomenon of slack flow in the downhill pipeline. According to all kinds of operating conditions, it may appear via the large gap areas. Based on VOF multiphase flow model numeral simulation was built to research the influence of slack flow features for different angles, different pipe diameters and flow velocity. Through the computer analysis, it elaborated the influence factors of the discontent flow phenomenon for product oil in a large gap location. The results provided theoretical guidance for the design of transport pipeline in mountainous areas.