Diffraction_enhanced imaging by using two crystals has been recently developed i n x_ray topogrphy station in Beijing Synchrotron Radiation Facility. In this art icle the analysis for the influence of the thermal expansion of the first crysta l on the diffraction enhanced imaging is presented. The conclusion is that the i nfluence of the thermal expansion of the first crystal can be avoided and much b etter images can be obtained when the axes of crystals are placed vertically to the polarization plane of the synchrotron radiation. The experimental results co nfirmed this analysis.