The formation of laser induced periodic surface structure (LIPSS) on the surface of molybdenum is investigated by using 400 nm and 800 nm femtosecond lasers. In the experiment, the influences of the processing parameters, specifically pulse energy, central wavelength, pulse overlap number and the fabricating ambient on the spatial period of the LIPSS are examined. In the ambient of water, ripple-like LIPSS on molybdenum with subwavelength spatial period of only around 160 nm is fabricated successfully by using 400 nm femtosecond laser with optimized processing parameters. Besides, an improved model is proposed to account for the LIPSS formation in the water. In this model, two kinds of interference mechanism, including the interference between the incident laser and the excited surface plasmon polaritions, and the other between surface plasmon polaritions propagating opposite directions, which causes standing wave, compete with each other. The improved model agrees well with the experimental phenomena, which is of great significance for fabricating nano-structure on the surface of metals.