The invention discloses a wide-width off-axis three-reflection-mirror optical system adopting a free curved surface, and belongs to the technical field of optics. A conventional off-axis three-reflection optical system adopting a free curved surface has a small field angle, and the adopted totally unbalanced free curved surface has large processing difficulty. The system is a wide-width off-axis three-reflection mirror optical system taking a primary reflection mirror as a free curved surface, and comprises the primary reflection mirror, a secondary reflection mirror, a third reflection mirror and a aperture diaphragm, wherein the aperture diaphragm is overlapped with the secondary reflection mirror; incident light is reflected to the secondary reflection mirror through the main reflection mirror, reflected to the third reflection mirror through the secondary reflection mirror, and then reflected through the third reflection mirror, so that an image is formed; the main reflection mirror is a free curved surface which is symmetrical to a meridian plane, so that the manufacturing difficulty of the system can be reduced; and at the same time, the system has a lager rectangular view field, so that wide-width imaging can be realized.