The unpolarizing beam splitter is a critical beam-splitting element in space optics. A high-splitting-ratio unpolarizing beam splitter was designed and fabricated in this study to achieve a stable output of a 780 nm band laser with a large splitting ratio. First， the double-sided unpolarizing film system was simulated and designed using TFCalc. Next， unpolarizing beam splitter samples were fabricated using ion beam-assisted deposition. Subsequently， the samples were measured and characterized using transmission electron microscopy and spectrophotometry， and the actual film structure and transmission spectrum were obtained. The spectral results showed that the transmittance of the beam splitter was close to 98%， and the transmittance deviation |T_s -T_p| was lower than 0.3%. Finally， the actual performance of the beam splitter was experimentally evaluated. The experimental results show that the transmittance of the beam splitter is close to 98% in the target wavelength band， and the transmittance deviation |T_s -T_p| is lower than 0.2% when the polarization direction changed in a broad range. The transmittance fluctuation is lower than 0.12% at 772-792 nm. The 10 h long-term results indicated that when the average time is 100 s， the Allan variances of the split ratio and transmittance are 1.4×10^-3 and 4.12×10^-5， respectively. The proposed unpolarizing beam splitter exhibits good depolarization performance and can be directly applied to precision measurement fields， such as optical test metrology and quantum-sensing detection.