Miniaturized coherent light sources on the nanoscale are highly desired for on-chip photonics integration. However, when approaching the diffraction limit, the sub-wavelength-scale all-dielectric lasers are difficult to realize due to the trade-off between lasing performance and physical size. Especially for a thin-film laser, usually an externally complex cavity is required to provide the necessary optical feedback. Herein, we successfully shrink the MAPbBr3 perovskite thin-film laser to sub-wavelength scale (300 nm) with simplified cavity design using only an ultraviolet glue layer and a quartz glass. The morphology quality and the gain properties of the film are enhanced by introducing ionic liquid. Consequently, the stable and low-threshold single-mode laser with a highly linear polarization degree of 78.6% and a narrow line width of 0.35 nm is achieved under two-photon excitation. The excellent single-mode laser with sub-wavelength scale and ultrasimplified structure could provide a facile and versatile platform for future integrated optoelectronic devices.