A new tunable orthogonal grating consisting of two weakly modulated subwavelength gratings perpendicularly to each other is presented. Using the rigorous coupled-wave analysis (RCWA), the spectra are calculated and the characteristics of guided-mode resonance for transverse electric (TE) and transverse magnetic (TM) polarized light is analyzed. It is demonstrated that a wide-range tunability of such structure is obtained via varying the spacing between the gratings (the distance between the two resonant wavelengths is continuously tunable from 0 to 62.7 nm). For the normally incident waves of both TE and TM polarized light, the structure turns out to have the same spectrum and resonance peaks, and it is the same as the overlay result of parallel double grating. For the obliquely incidence of TE/TM polarized light, the separation of the resonant peaks appears. At the same time, some specific wavelengths are maintained with the increase of the incident angle, such as the resonance peaks of 582.3 nm and 590.8 nm for incident plane P1, and those of 590.8 nm and 612.4 nm for P2.