A computational study is performed for exploring flow and flame dynamics of a high-pressure hydrogen/oxygen coflow jet with the effects of post thickness and moment flux ratio. A two-dimensional model with a splitter plate, which represents a post configuration of an injector of rocket engines, is adapted to fully resolve the combustion flow field. The compressible Navier-Stokes equations are solved with a detailed chemical kinetic mechanism in a manner of direct numerical simulation. The result shows that the post thickness largely affects the temperature distribution in a recirculation region established behind the post. The temperature distribution is determined with the amount of incoming high-temperature combustion gas and unburned hydrogen gas, which significantly changes with the post thickness. The effect of the momentum flux ratio clearly appears in the case of thicker post configuration, while in the case of thinner post configuration no major differences are identified for all the momentum flux ratio. The study shows a tendency that thicker post geometries with smaller J numbers provide lower temperature fields in the recirculation region behind the post, thus preliminarily indicating some difficulty of maintaining a flame anchoring in the recirculation region.