Beam-splitting photodetectors in optical communication network monitoring systems are used to detect the flow of optical signals and have a large number of applications in the field of communication and signal detection. In order to meet the device usage requirements, a beam-splitting photodetector film is developed based on ion beam aided deposit(IBAD) technology. Its transmission and reflection energy splitting ration is 50:50 in the wavelength range of 1520 ～ 1620 nm. The optical constants of coating material are analyzed and calculated. By establishing the error function, the error of optical constant of coated material is optimized and reduced. In the design of beam-splitting photodetector film, the spectrum curve is smoothed by adding the dummy layer, and the sensitivity of the layer thickness is reduced. As a result, the difficulty of the preparation process is reduced. The transmittance and reflection splitting ration of the prepared beam-splitting photodetector film in the wavelength range 1260 ～ 1620 nm is less than 50:50(±0.5%), which meets the requirements of the parameters.