Abstract: VSOP code is widely used in the engineering design of pebble bed high temperature gas-cooled reactor. In order to control the reactivity, some absorbers are laid in the holes of the side reflector of the core. Because of the limitation of physical calculation method, VSOP code isn’t capable to calculate the absorber value, so other external deterministic codes are needed. In the process of coupling these codes, approximation of the geometric structure as well as merger and conversion of the cross section are involved, which may introduce additional error. In this paper, the Monte Carlo code was adopted to build the detailed model of the core, including accurately describing the pebble bed structure of the active core, the holes of the side reflector, as well as the shape and position of the absorbers. Then based on the same reactor condition, the absorber value was calculated respectively with the deterministic coupled code and the Monte Carlo code. It is shown that the calculation result from the deterministic coupled code is accurate and conservative for the engineering design.