The utility model relates to a transmission case, and in particular relates to a transmission case assembly of a harvesting machine. The transmission case assembly comprises a case body, wherein a first shaft, a second shaft, a third shaft, a fourth shaft, a fifth shaft and a differential gear are arranged in the case body; and the five shafts are gear shafts which are meshed with gears on the differential gear. The transmission case assembly is characterized in that a first shaft gear is arranged on the first shaft; a second shaft bull gear, a second shaft low-gear gear and a second shaft reverse-gear gear are arranged on the second shaft in sequence; a third shaft shifting slide gear I, a third shaft shifting slide gear II, a third shaft middle gear, a third shaft bull gear and a third shaft pinion are arranged on the third shaft in sequence; a speed reducing pinion, a fourth shaft shifting slide gear I, a fourth shaft shifting slide gear II and a fourth shaft shifting slide gear III are arranged on the fourth shaft in sequence; a speed reducing bull gear is arranged on the fifth gear; and a gear shift fork assembly is respectively arranged at the shifting slide gears of the third shaft and the fourth shaft. The transmission case assembly of the harvesting machine has the beneficial effects that six forward gears and one reverse gear can be transformed, and the requirement for speed of the harvesting machine are met.