Carbon fibers were multiphase polycrystalline materials. To study the influence of microcrystalline structure characteristics on thermal conductivity， the characteristics of the microstructure of carbon fiber， the relationship between thermal conductivity and microstructure was studied by XDR. The results highlight that the thermal conductivities increases with the increase of lateral size （L_a）， stacking thickness （L_c）， average stacking layers （N） of graphitic microcrystals and decreases with the increase of porosity （V_p）. Based on the peak fitting information of Raman spectra， the graphitization degree of carbon fibers was analyzed. The results reveal that the higher the graphitization degree of carbon fiber， the higher the thermal conductivity.