An optical system, comprising: a display screen, and a third lens, a second lens and a first lens arranged successively along a propagation direction of incident light, an effective focal length f1 of the first lens and an effective focal length f2 of the second lens are both greater than an effective focal length f of the optical system, the third lens, second lens and first lens are arranged at a light exit side of the display screen, in the propagation direction of the incident light emitted from the display screen, and used to project the incident light into the human eye for imaging, wherein the first lens and the second lens are glued together, the two glued surfaces of the first lens and the second lens are both Fresnel surfaces, the effective focal length f1 of the first lens is set to 20mm ≤ f1 ≤ 40mm; the effective focal length f2 of the second lens is set to 20mm ≤ f2 ≤ 40mm; and the effective focal length f3 of the third lens is set to -100mm ≤ f3 ≤ -50mm, the first lens, the second lens and the third lens are located on a same optical axis, the first lens comprises a first surface and a second surface, and the second lens comprises a third surface and a fourth surface, the second surface and the third surface are arranged adjacent to each other and both are set as Fresnel surfaces, and the first surface and the fourth surface are both convex surfaces, the thickness h1 at the center of the first lens is 2mm ≤ h1 ≤ 4mm; the thickness h2 at the center of the second lens is 3mm ≤ h2 ≤ 5mm; the thickness h3 at the center of the third lens is 2mm ≤ h3 ≤ 4mm, a spacing T1 is provided between the first lens and the second lens, and the spacing T1 is set to 0.1mm ≤ T1 ≤ 0.5mm; a third spacing T3 is provided between the third lens and the display screen, the third spacing T3 is set to 8mm ≤ T3 ≤ 15mm; a spacing Y is provided between the second lens and the third lens, and the spacing Y is set to 2mm ≤ Y ≤ 4.5mm; the second lens is kept still, the size of the light passing aperture of the third lens is changed by adjusting the spacing Y between the third lens and the second lens, so that the smaller the spacing Y is, the larger the aperture x of the third lens is, the relationship between the spacing Y and the aperture x of the third lens is as follows: Y = ax + b, where -0.8 ≤ a ≤ -0.39, 30 ≤ b ≤ 35. a display screen, and a third lens, a second lens and a first lens arranged successively along a propagation direction of incident light, an effective focal length f1 of the first lens and an effective focal length f2 of the second lens are both greater than an effective focal length f of the optical system, the third lens, second lens and first lens are arranged at a light exit side of the display screen, in the propagation direction of the incident light emitted from the display screen, and used to project the incident light into the human eye for imaging, wherein the first lens and the second lens are glued together, the two glued surfaces of the first lens and the second lens are both Fresnel surfaces, the effective focal length f1 of the first lens is set to 20mm ≤ f1 ≤ 40mm; the effective focal length f2 of the second lens is set to 20mm ≤ f2 ≤ 40mm; and the effective focal length f3 of the third lens is set to -100mm ≤ f3 ≤ -50mm, the first lens, the second lens and the third lens are located on a same optical axis, the first lens comprises a first surface and a second surface, and the second lens comprises a third surface and a fourth surface, the second surface and the third surface are arranged adjacent to each other and both are set as Fresnel surfaces, and the first surface and the fourth surface are both convex surfaces, the thickness h1 at the center of the first lens is 2mm ≤ h1 ≤ 4mm; the thickness h2 at the center of the second lens is 3mm ≤ h2 ≤ 5mm; the thickness h3 at the center of the third lens is 2mm ≤ h3 ≤ 4mm, a spacing T1 is provided between the first lens and the second lens, and the spacing T1 is set to 0.1mm ≤ T1 ≤ 0.5mm; a third spacing T3 is provided between the third lens and the display screen, the third spacing T3 is set to 8mm ≤ T3 ≤ 15mm; a spacing Y is provided between the second lens and the third lens, and the spacing Y is set to 2mm ≤ Y ≤ 4.5mm; the second lens is kept still, the size of the light passing aperture of the third lens is changed by adjusting the spacing Y between the third lens and the second lens, so that the smaller the spacing Y is, the larger the aperture x of the third lens is, the relationship between the spacing Y and the aperture x of the third lens is as follows: Y = ax + b, where -0.8 ≤ a ≤ -0.39, 30 ≤ b ≤ 35.