In seismic numerical simulation, the perfectly matched layer (PML) boundary can attenuate the seismic wave effectively, and the snapshots and record have no boundary reflection interferes.The authors further analyzed the attenuation mechanism of split perfectly matched layer (SPML).By wave equation simulation, the authors analyzed the problem as to how the parameters affect absorption and attenuation characteristics of SPML boundary, such as source dominant frequency, spatial grid spacing and medium speed.As a result, the authors obtained the results of PML attenuation based on different conditions.Through analyzing and comparing the results, the conclusions have been reached that source dominant frequency has no direct influence on PML attenuation, spatial grid spacing is in inverse proportion to the PML attenuation, and high-speed medium slows down PML attenuation effect.By Marmousi2 test, the authors confirmed the validity of the conclusions.The selection method of SPML parameter in complex model or different scales of numerical simulation was given, which lays a foundation for quantitative selection of SPML boundary parameters in numerical simulation.