An optical system and a projection device are provided. The optical system includes a display unit, a first lens group, a second lens group, and a reflector in sequence along a transmission direction of light; both the first lens group and the second lens group have positive focal powers; the optical system satisfies the following relationship: 0.01≤|φ100|≤0.02 and 0.005≤|φ200|≤0.015; φ100 represents a focal power of the first lens group, and φ200 represents a focal power of the second lens group.