Abstract: Based on a series of numerical calculations,the behavior of flow field in obstruct ed round buoyant vertical jet is summarized and analyzed.Based on the axial line velocity distribution,the flow after the disc can be divided into three regions:i e recirculation region,transitional region and self similar region.The laws of the length of recirculation region varying with Froude number F₀,port diameter D/d and the distance H/d of the disc from the port are got.The velocity distribution charts in cross sections and the least length from the disc needed for reaching self similar area in different Froude number under three different conditions of H/D=1,D/d=2,6 and H/D=3,D/d=2 are also obtained.And it is found that Froude number is the key factor to decide which flowing pattern in the field.The pressure distribution changing in the flow area for three kinds of flow patterns is analyzed and it is found that the magnitude and location of the third negative pressure center caused by the increase of Froude number are closely related to flow patterns.