The formation of one dimensional periodic subwavelength (~250 nm) ripple structures on the surface of Si-based Cr film was investigated by irradiation of blue femtosecond laser pulses (1 kHz, 50 fs, 400 nm) under different ambient air pressures of 105 Pa (or 1atm) and 10－3 Pa. The effects of the experimental parameters such as the incident laser energy fluence, the sample scanning speed and the film thickness, on the development of the surface ripple structures were compared. The results demonstrate that the quality of the surface structures can be effectively improved either by the low air pressure of 10－3 Pa or by reducing the thickness of Cr film (100 ~25 nm), however, the uneven ablation may be appeared when the Cr film is too thin (≤ 25 nm). Moreover, with decreasing the laser energy fluence or increasing the scanning speed, the spatial period of the ripple structures tends to become smaller, accompanied by the nanowires growth inside the grooves of the ripple structures.