An optical system and a focusing structure for an infrared thermometer are provided. The optical system includes a focusing ocular barrel (100) and an objective focusing ring (300), provided at the rear end of an optical probe. The shift of the ocular (101) and the objective (201) can be controlled. The rear end of the probe includes a cap (507), which can be used for locking the detective image distance of the objective (201) and sealing and protecting the portion of the focusing operation.