Nonlinear frequency conversion is one of the key research contents in nonlinear optics. Using the external high voltage electric field poling technology to modulate the second-order nonlinear coefficient of nonlinear photonic crystals, quasi-phase-matching technology is commonly used to obtain efficient nonlinear frequency conversion. As the carrier of second-order nonlinear optical effects, optical superlattice is one of the important research topics in the field of nonlinear optics. In order to explore the advantages of different superlattice structures, more and more superlattice structures are applied to optical materials to generate harmonics of different wavelengths. Designing excellent optical superlattice structures to perform nonlinear frequency conversion has become the best solution to meet the needs of scientific research.Fractal superlattice structure has the advantage of relatively sparse arrangement in the real space and dense distribution in the reciprocal space. Therefore, fractal superlattice structure can provide highly efficient nonlinear interactions, and its advantage is more reflected in the higher-order frequency conversion. In this paper, Gosper fractal and Z fractal superlattice structures are introduced into nonlinear photonic crystals. In real space, Gosper fractal or Z fractal superlattice structure is composed of one continuous line. These two fractal superlattices can provide the complex spatial structure and rich reciprocal vectors. For Gosper fractal, it has six-fold rotational symmetry in the reciprocal space. It not only has the overall fractal dimension, but also has the boundary fractal dimension. For Z fractal, it has the periodic, quasi-periodic and fractal characteristics. The overall pattern gives the translational symmetry.The distributions of their reciprocal vectors are simulated by two-dimensional Fourier transform. The quasi-phase-matching harmonis in LiNbO₃ nonlinear photonic crystals with these two superlattices are theoretically analyzed. Moreover, their diffraction experiments were carried out. The reciprocal space of the Gosper fractal superlattice can still coincide with the original pattern after the overall rotation of 60°. It means that the same harmonics can be achieved along six different directions. In the cascaded third harmonic generation, the outputs of 711.3 nm, 554.7 nm and 491 nm are obtained at different fundamental wavelengths, and their deviation angles are calculated theoretically. In the reciprocal space of Z fractal superlattice, we can make full use of the high-order transverse reciprocal lattice vectors. Second harmonics of the incident wavelength range of 1.402~1.430 μm are realized, with a minimum wavelength spacing of only 1 nm.In conclusion, the advantage of the Gosper fractal superlattice structure is the same diffraction points along six different directions. Combining the advantages of the traditional hexagon and hexagonal lattice periodic structures, it not only has high conversion efficiency, but also can realize the same multiple harmonics along six different directions. These are of great significance to the research and development of multi-channel optoelectronic integrated devices. For Z fractal superlattice structure, the reciprocal vectors are densely distributed, by which it is helpful to realize the harmonics outputs of quasi continuous wavelengths and improve the practicability of quasi-phase-matching technology. Finally, 3D Z fractal is introduced into nonlinear photonic crystal, which provides data support for femtosecond laser preparation of 3D Z fractal superlattice structure crystal. Compared with 2D crystal, the quasi-phase-matched second harmonics in 3D Z fractal crystal may be significantly improved.