The invention relates to a diggingbolt group cutting device, in particular to a speed reducer of the diggingbolt group cutting device to solve the problems of the defects of a cutting speed reducer in an existing cutting device. The speed reducer structurally comprises an upper shell, a lower shell, a left supporting shell, a right supporting shell, an upper gear case, a lower gear case, a left planetary gear case, a right planetary gear case, cutting blocks, a cutting gland, a left supporting shell gland, a right supporting shell gland, a five-level left planet carrier, a five-level right planet carrier and hubs. A left power input bevel gear shaft and a right power input bevel gear shaft are connected with the left planetary gear case and the right planetary gear case through a driven bevel gear, a spline shaft drive helical gear and a transmission shaft respectively. Cutting rollers are connected to the two sides of the left planetary gear case and the right planetary gear case to form double-motor drive, an overall-width cutting mechanism of a diggingbolt group is more stable and reliable in work, the technical support is provided for mining machine serialization development in China, the technical reserve is provided for heavy tunneling devices, and the technological level of the coal mine tunnel tunneling device field in China is improved.