The invention relates to an illuminating system and a focal length adjustment device thereof, wherein the focal length adjustment device is used for adjusting the distance between a light source of the illuminating system and a light reflecting cup, and comprises a guide rail fixing plate which is provided with a guide rail parallel to a main optical axis of the light source; the guide rail is internally penetrated with a sliding rod which can relatively slide, one end of the sliding rod is connected with the light reflecting cup, and the other end is connected with a driving device; the driving device comprises a motor fixing plate connected with the sliding rod, and the motor fixing plate is provided with a motor; a motor output shaft is provided with a cam which can rotate correspondingly to the motor fixing plate; the driving device also comprises a focusing component; and an elastic part is positioned between the guide rail fixing plate and the motor fixing plate. The invention can avoid the relative motion between the motor output shaft and a motor shell part, thus reducing the damage for the motor output shaft; furthermore, the distance between the motor and a lamp housing can be adjusted by a focusing nut.