Femtosecond laser-induced periodic surface structure (LIPSS) has the potential applications in tunable thermal source, tribology, super-hydrophilicity, super-hydrophobicity, marking and so on. LIPSS formation and mechanism on tungsten with a 800 nm femtosecond laser are investigated. The electromagnetic energy distribution on random rough surface after the first laser pulse and the energy distribution of electromagnetic field induced by low-spatial-frequency LIPSS after 20 pulses are simulated by Sipe interference model and finite-difference time-domain (FDTD) method. The formation mechanism of low-spatial-frequency and high-spatial-frequency LIPSS (HSFL) is disclosed. The evolution of surface morphology and the phenomena of spatial period decreasing with the increase of laser pulse number are also investigated.