Dark-field scattering detection is one of the main methods of non-patterned wafers detection. The incident light shapes and scattered light collection system will directly affect the detection performance of the system. Therefore，the simulation analysis of the defect scattering field and the optical path system are the basis of the system design. In this paper，a simulation system for defect detection of non-patterned wafers surface based on dark-field scattering is established by Finite-Difference Time-Domain（FDTD）method，including particle scattering simulation on the smooth wafer surface，roughness scattering simulation on the wafer surface and performance simulation of collecting optical path system. In this paper，the characteristics of scattering field under different laser incident angles，polarization states and defect sizes are analyzed. The corresponding relationship between spot diameter and detection limit is explored，and the optical path system is designed and simulated according to the scattering characteristics. The optical path system designed according to the simulation results has the characteristics of compact structure and high collection efficiency（the average collection efficiency is 77.65%），which provides a reference for the surface defect detection of non-patterned wafers.