A grating monochromator combined by a cylindrical mirror and a straight groove toroidal grating is discussed. The cylindrical mirror is enployed to form the monochromator system of astigmatic coma-free. The monochromator has a large horizontal collection angle. The calculated results show that the line broadering mainly comes from defocusing of the toroidal grating. The expected performance of the monochromator could be obtained. The optical designing process of the monochromator and the VUV beamline confiquration for synchrotron radiation are also considered in detail.