Abstract: A plastic scintillation fiber detector with a multi-layer array structure could be used for the direct and rapid determination of ⁹⁰Sr in soil. The measuring accuracy and the resistance to γ-ray interference of the detector depend on the specific structure design and signal processing mode. The structure of the detector was optimized by modeling and simulation using the Monte Carlo method. The recommended scintillation fiber array is composed of five layers. The fourth layer is the polymethyl methacrylate (PMMA) absorbing layer whose thickness is no less than 6 mm, while the other layers are two-clad square plastic scintillation fiber array with a thickness of 1 mm. The γ discrimination is improved by the coincidence of the signals from the first three layers.