In order to identify the two characteristic lengths of breakup process of low-pressure jets, the relationship between different diameters and angles of entrance of jet nozzle on the breakup of low pressure jet under different pressures was studied. An experiment was conducted to investigate the core lengths and breakup lengths of a round water jet. The flow, ranges and ending raindrop diameters were measured under low jet velocity. The results showed that with the increase of nozzle diameter, the flow, ranges, ending raindrop diameters, core lengths and breakup lengths became larger. With the increase of nozzle cone angle, the flow became smaller, the ranges increased at first then decreased, the ending raindrop diameters increased and the core lengths decreased, but the breakup length decreased at first then increased. The ranges and the atomizing condition showed that the diameter of nozzle of 5mm and cone angle of nozzle of 45° were the best. Correlation equation was proposed based on the analysis of effect of Re and We on the core length and breakup length.