The field distributions of the focal region, where the beam with a certain incident angle is focused by the spherical reflector, are theoretically calculated and measured. The results give evidence that the focusing system of the spherical reflector in grating spectrometer for millimeter waves (25~100GHz) strongly limits its resolving power. In- creasing the diffraction angles of the diffracted beam from a grating is an effective way to abate or eliminate this limit.