The fabrication of nanostructures beyond the diffraction limit has been the focus of nanotechnology research. Scanning probe microscopy (SPM) has attracted the attention of researchers for the detection and manufacture of nanostructures. Here, a nanosecond laser irradiated a cantilevered scanning near-field optical microscopy (SNOM) tip and directly wrote subwavelength nanostructures on Au nano-film, without the assistance of a mask or vacuum atmosphere. This method was stable and reproducible for long-term use. The in situ morphology detection was conducted after the writing process by atomic force microscope (AFM). A feature linewidth of approximately 83.6 nm (< λ/6) was confirmed using scanning electron microscopy (SEM). Linewidth of (167.8 ± 6.6) nm was reproduced stably. Theoretical calculations revealed that the elliptical heat distribution under the SNOM tip generated different linewidths when the tip scanned vertically and horizontally. It also interpreted the influential mechanism of single-pulse energy. The simulated linewidths were consistent with the fabricated linewidths. According to the elemental analysis by energy dispersive spectrometer (EDS), the mechanism of this method can be interpreted as melting of the Au nano-film instead of oxidation. Owing to its high positioning, machining accuracy, and instantaneous energy, this technology is considered convenient and economical for nanostructure fabrication and is proposed to be applied in nanolithography on multiple materials in the future.