Speed reducer based on split type shell

The invention provides a speed reducer based on a split type shell. The speed reducer based on the split type shell comprises a main shaft, planetary gears, crankshafts, cycloid bearings, a shell bodyand cycloid pin wheels, wherein the shell body is of a hollow cylindrical structure, and the main shaft is coaxially arranged in the geometric center of the shell body; the three crankshafts are arranged, in parallel to the main shaft, on a circle taking the main shaft as the center with equal radians; a planetary gear is fixedly arranged at the upper end of each crankshaft, a gear is fixedly arranged at the upper end of the main shaft, and the gear is meshed with all the planetary gears; each crankshaft is fixedly sleeved with two cycloid bearings, the main shaft is sleeved with two cycloidpin wheels, the cycloid pin wheel is located at the position of the crankshaft and is provided with a hole, the cycloid pin wheels are provided with holes at the positions of the crankshafts, the crankshafts penetrate through