Hinge technology is used to study focusing behavior of a sagittal focusing crystal monochromator. A mathematical model describing the relationship between actuator displacement and dynamic bending radii has been established by studying geometry parameter of the hinge. Influential factors on resolution of the bending radii were analyzed. The model has been successfully applied in developing a sagittal focusing crystal monochromator for the SSRF (Shanghai Synchrotron Radiation Facility) project. The testing result indicated that focusing spot of the sagittal focusing crystal monochromator achieved satisfactorily the design requirement.