The invention discloses an optical slit with a submicron thickness and with sapphire as a substrate. According to the slit, a submicron-thickness nickel-chromium alloy matt film layer is firstly plated on the incident plane of the sapphire sheet substrate to form a slit pattern, and then antireflection films in corresponding bands are plated on the incident plane and an emergent plane respectively. Thus, the spectral transmittance in a transparent slit area is larger than 97% in a 0.95 to 2.50 micron spectral range, and the average spectral transmittance in an opaque region is lower than 0.1%. The width and the shape of the slit are adjustable, and the linear precision can reach 1 micron. The thickness of the slit is only 200 to 300 nanometers, stray light can be effectively removed, the slit structure is concise, the positioning precision is high, and the slit can be applied to a short wave infrared instrument such as a short wave infrared imaging spectrometer.