<p>Seismic traveltime is a crucial seismic attribute that directly influences the computational accuracy and efficiency of various seismic processing and interpretation methods. The Fast Marching Method (FMM) is an accurate and stable finite-difference approach for calculating seismic traveltime. However, when applied to three-dimensional models, its computational efficiency becomes a major limitation. To address this issue, this paper proposes an efficient traveltime computation algorithm that integrates coarse-grid interpolation with the FMM. The proposed method first computes the global traveltime field on a coarse grid using FMM, then refines the results to a fine grid across the entire domain via cubic spline interpolation. Both theoretical analysis and numerical experiments demonstrate that the proposed approach preserves the unconditional stability of the FMM while significantly reducing memory consumption and computational time by combining coarse-grid computation with interpolation-based refinement.</p>