Based on a sandwich structure and adiabatic coupler, a polarization-independent optical power splitter with a designable splitting ratio is designed to achieve power distribution for the 1550 nm wavelength optical signal with a designable splitting ratio. By adjusting the refractive index of the sandwiched middle layer material SiN_x, the splitting ratios of the transverse electric (TE) and transverse magnetic (TM) polarization modes are made equal at the same wavelength, and a polarization-independence function is realized. Next, the designable splitting ratio function is obtained by varying the asymmetry of the waveguide gaps in the adiabatic coupler. The three-dimensional finite-difference time-domain method is used for modeling and simulations. The results reveal that the coupling length of the proposed device is only 7 μm. The device can achieve designable splitting ratios ranging from 0.50 to 0.95 and simultaneously support the TE and TM polarization modes. The insertion loss value is lower than 0.31 dB. A 100 nm bandwidth can be obtained when the tolerance of the splitting ratio is within ±0.01. The proposed approach is potentially applicable to future photonic integrated circuit systems.