The invention discloses a manual-shift speed reducer of a crowned-gear clutch for large-size petroleum drilling machines, which comprises a case, an input shaft system, an output shaft system, a shift operating mechanism, a lubricating system, a ventilator system and a measuring rod system. A lower case, a middle case and an upper case are assembled from bottom to top vertically to form the case, the input shaft system, the output shaft system, the shift operating mechanism, the lubricating system, the ventilator system and the measuring rod system are arranged in the case, the input shaft system is used for inputting power and realizing high revolution speed, the output shaft system is used for outputting torque and realizing low revolution speed, and the shift operating mechanism is used for changing the transmission ratio of the input shaft system to the output shaft system. The manual-shift speed reducer of the crowned-gear clutch for the large-size petroleum drilling machines has wide applicable range of environmental temperature, is capable of running on open sites full time, is wide in applicable range of environmental temperature and has the advantages of high bearing capacity, long service life, large output torque, high transmission efficiency, low operation noise and vibration of the case, fine tightness, high exchangeability, convenience in installation, flexibility and stability in shift.