Power splitter with multi-mode interference coupler structure has many advantages, such as large bandwidth and better manufacturing robustness, and has received much attention for a long time. Conventional power beam splitters usually use algorithms or numerical simulation to achieve a single beam splitting ratio; if the circuit has the requirement for power, the structural parameters of the device need changing and recalculating. In order to improve the utilization rate of power splitter in photonic integrated circuit and meet various demands for different optical paths, an ultra-compact tunable power splitter based on phase change material In ₂Se ₃with a 1×2 multimode interference coupler structure is proposed in this paper. The device consists of an input waveguide, a coupling region, and two output waveguides with a coupling region of only 2.4 μm× 3.6 μm in size, which contains several circular holes of the same size and is filled with SiO ₂. The number and location of circular holes in the coupling region are optimized by direct binary search algorithm, making the device achieve different power splitting ratios by using only the high refractive index contrast variation between the two crystalline states ( αand β) of In ₂Se ₃without changing any other structural parameter. In a wavelength range of 1540–1560 nm, three splitting ratios of 1∶1, 1∶1.5 and 1∶2 are achieved by this device, and the insertion losses of these three beam splitting ratios are less than 0.27, 0.13 and 0.17 dB, respectively. In addition, the robustness and balance of the device are analyzed and discussed, and compared with those of the power splitter of the same size designed by SOI platform and several power beam splitters reported in recent years, demonstrating the compact structure and simple regulation of this power splitter based on the phase change material In ₂Se ₃, its good robustness, and its possibility of application on photonic integrated circuits.