The invention provides a near-infrared long-working distance monochromatic microscopic objective lens with NA=0.4. The objective lens is successively formed by a first component (G1), a second component (G2) and a third component (G3) from the objective space to the image space. The first component (G1) is a vacuum observation window sheet (P1). The second component (G2) is formed by a first lens (L1), a second lens (L2) and a third lens (L3) and is provided with the positive focal power. The third component (G3) is a fourth lens (L4) and is provided with the negative optical power. According to the invention, the working distance of the objective lens can be 22mm when there is no window sheet. The objective lens can work in 700-2300nm wave length. While the long working distance and high definition are achieved, optimal adjustment and compensation can be performed on aberration caused by different thicknesses of vacuum window sheets. The whole objective lens is formed by single-sheet lenses made of the same kind of optical glass, so the structure of the system is simple and the objective lens can be widely used in precision observation for microcosmic objectives with micrometer scales in a glass air room.