In this paper, a two-dimensional photonic crystal is used to design a miniaturized, dense, scalable eight-channel multiplexer structure. The two-dimensional photonic crystal is formed by periodically arranging silicon dielectric columns in the air using a triangular lattice. The linear main waveguide, download cavity, and output waveguide structure are designed, and the coupling effect between the cavity and waveguide is used to select the wavelength. The designed cavity is composed of one inner and four outer columns, and it has a simple structure. The cavity mode can be adjusted through the diameter of the inner and outer columns to realize multiwavelength selection and the scalability structure. A quasi-dense wavelength division multiplexing structure is realized. The multiplexing of eight wavelengths, including 1542.2, 1544.2, 1546, 1548.2, 1550, 1552, 1554.4, and 1557.6 nm, achieve 2.2 nm of the average wavelength interval, and the insertion loss is -0.5, -0.25, -0.25, -0.7, -0.25, -0.5, -0.1, and -0.1 dB. respectively. The maximum channel crosstalk is -12 dB. The device size is 19.8 μm × 11 μm. It achieves miniaturization, high transmission, and high isolation. It also lays a foundation for optical integration. The design and analysis are done with the finite element method using COMSOL software.