A non-contact cascaded dual-layer sub-wavelength grating is presented. It can broaden the spectral peak linewidth, modify the spectral lineshape and improve the optical variable effect of basic sub-wavelength gratings. It is a kind of security grating configuration with potential for achieving application in industry. The mechanisms for the broadened linewidth and improved lineshape are analyzed and validated with the vector diffraction theory. The structural parameters of dual-layer gratings with special requirements of change in color are optimized and their optical variable characteristics are also studied. The results show the spectral lineshape of dual-layer gratings cascaded in phase is similar to that of basic gratings with a linewidth twice that of basic gratings; the out-of-phase arrangement yields lineshape exhibiting broadened peaks and narrower bases with a wider linewidth. Dual-layer gratings always show optical variable effect when tilted at traditional or nontraditional incident illumination and the optical variable velocity of the former is the quicker than that of the latter which can be used to design and fabricate anisotropic optical variable image devices.