Spin waves (SWs) have tremendous application potential in wave-based computation utilizing a broad frequency spectrum spanning from the gigahertz to terahertz ranges. Like optical and other electromagnetic waves, SWs also promise to usher in a new era of parallel data processing with low-power consumption without Joule heating. However, this potential is undermined by the lack of investigation on multichannel networking and operation on single chips under a uniform bias magnetic field. This study proposes a multifrequency SW propagation based on shape anisotropy in microstructured rectangular waveguides made of yttrium iron garnet (YIG). The width-dependent transmission properties of magnetostatic surface SWs in the YIG waveguides were experimentally demonstrated. We revealed that the smaller width of the waveguide results in lower SWs frequency due to the demagnetizing field along the width direction. Multifrequency SW propagation was demonstrated in a device where three waveguides with widths of 10, 20, and 100 μm were connected to the common antennas. SWs propagation with the frequencies of 1.98, 2.11, and 2.18 GHz have been transmitted under a uniform bias magnetic field. Furthermore, we investigated SWs transmission in a device where three waveguides with different widths were interconnected at their ends and under one side of the antenna. We observed that the interconnected waveguides result in a single resonant frequency with flat band transmission because the whole waveguide structure is considered a single magnetic body. The results presented here provide guidelines for complex networks in frequency-division multiplexing operation.