Amid the rising demand for high-performance computing, photonic integrated circuits are increasingly overcoming the conventional two-dimensional barriers, transitioning toward more flexible multilayer structures. To fulfill this aim, we have engineered a wide-bandwidth, multilayer tunable power splitter that enables the transmission of information along the vertical direction while allowing for flexible allocation of optical power. The power splitter is constructed with an asymmetric coupler, complemented by a grating structure. The design of the coupler has been refined through optimization employing the particle swarm algorithm, while the grating structure has undergone optimization via the direct binary search algorithm. The simulation results indicate that the power divider can achieve proportional regulation from 0.285 to 3.5 across the 1400 to 1700 nm wavelength spectrum, with insertion losses (ILs) consistently below 0.34 dB. Significantly, the IL is <0.21 dB at a 1:1 power ratio. This compact, low-loss, high-bandwidth tunable power splitter was designed to offer a new idea for the multilayer integration of microchips.