The diffraction geometries of asymmetric Johann crystal monochromator and asymmetric logarithmic-spiral crystal monochromator are analysed in detail. The formulas for calculating the distance L1 between focus of X-ray source and monochromator, the dispersion width b1, b2 of cross section of source and focusing line are given. According to the result of theoretical analysis, the adjustable elastically bent quartz crystal monochromator which is the approximate asymmetric Johann type is used in the construction of X-ray focusing camera.