To improve the performance of beamline for X-ray Absorption Fine Structure (XAFS) in National Synchrotron Radiation Laboratory(NSRL), an idea to reform the current double-crystal monochromator into a sagittal focusing crystal monochromator was proposed. An assembly crystal which cements the silicon strips with the tops of titanium stiffening ribs was developed to used as an imaging element of the second crystal of the sagittal focusing monochromator and to minimize the anticlastic curvature and lower the breaking risk of sagittal bent crystal. Based on the parameters of the NSRL-XAFS beamline, a model of assembly crystal was designed and its focusing properties were evaluated by finite element analysis and were measured with a long-trace-profiler. The results indicate that the Full Wave at Half Maximum(FWHM) of beam spot on the sample is decreased to 3 mm from 43 mm(without focusing) in horizon, and the photon density on the sample is increased by one order of magnitude. The results meet the requirements of data quality in current available light sources.