The invention discloses a focusing device in a long-focus and heavy-caliber optical system, relating to the technical field of optical machinery and aiming to solve the problems that the traditional focusing mechanism is difficult to determine the coaxial precision with optical axes of other fixed mirror groups during installation and has high difficulty in installation and adjustment. The devicecomprises an electric control displacement platform, a focusing mirror group, a sliding rail seat, a shifting fork and two cylinders, wherein the focusing mirror group is arranged in the sliding railseat; the shifting fork is arranged on the outer side of the sliding rail seat and is fixedly connected with the electric control displacement platform; the focusing mirror group is in threaded connection with the two cylinders; U-shaped grooves are respectively arranged at two ends of the shifting fork; the sliding rail seat is provided with two slotted holes; and the cylinders sequentially passthrough the slotted holes of the sliding rail seat and the U-shaped grooves of the shifting fork. The invention is widely applied to the optical system.