The invention relates to an underground optical cable connecting device which is characterized by comprising a metal protecting tube, wherein two ends of the metal protecting tube are respectively provided with sealed fastening devices which are used for connecting the optical cables; each of the sealed fastening devices comprises a front clamping nut which is sleeved on the end part of the metal protecting tube; a front sealing sleeve and a front clamping sleeve are arranged in an annular space between the front clamping nut and the metal protecting tube; the front clamping nut is connected with a connecting sleeve via a thread; the other end of the connecting sleeve is connected with a back clamping nut via the thread; the optical cable is connected in the end part of the metal protecting tube after passing through the back clamping nut and the connecting sleeve; a back clamping sleeve and a back sealing sleeve are arranged in the annular space between the back clamping nut and the optical cable; and the welding point of the optical fiber in the optical cable is arranged at the central part of the metal protecting tube. The underground optical cable connecting device simple in structure design, small in volume and convenient in assembly; the underground optical cable connecting device is cable of protecting the optical fiber from directly exposing in rigorous underground environment, reducing or avoiding the influence caused by the severe conditions to the optical fiber performance; therefore, the underground optical cable connecting device can be widely used for underground cable connection.