Abstract: Improper design of flow passage converter section in LWD ( logging while drilling) tools can cause local flow-field turbulence and result in serious local erosion of the tool, thus reducing its service life. It can also lead to large pressure loss of the tool and affect its applicability. For this reason, CFD (computational fluid dynamics) method was applied for the optimization design of a certain type of flow passage converter in an LWD tool. According to the full 3D numerical simulation and comparison of four design schemes, the main factors affecting the flow-field performance of the flow passage converter were thought to be the expansion angle and the continuity of the cross-sectional area of the internal flow passage. The optimal design has a smaller expansion angle, a more continuous cross-sectional area of the internal flow passage, a gentler decline in axial velocity, a minimum total pressure loss, and a more uniform flow velocity distribution in the flow field. The research results showed that the expansion angle and the discontinuity of the cross-sectional area of the internal flow passage were negatively associated with the uniformity of the flow field distribution in the flow passage converter and positively with the pressure loss. The difference between theoretical and experimental total pressure loss coefficients was not more than 0.076%, and the change trend was the same as the result of theoretical analysis. The research results can effectively serve as a theoretical basis for the optimization design of flow passage converters.