Disclosed is a pre-tightening device of coupled thrust ball bearings. The pre-tightening device of coupled thrust ball bearings includes a nut, a circular inclined section spring, and two clamping plates. The two thrust ball bearings are arranged on a transmission shaft back to back in a sleeving mode and the outer rings are all supported on a bearing seat; the inner rings of the two thrust ball bearings are fixed through the nut and a first shaft sleeve; the outer rings of the two thrust ball bearings are fixed through a bearing end cover, and a second shaft sleeve; a first clamping plate isarranged on the shaft shoulder of the transmission shaft; a second clamping plate is arranged on an inner ring of the thrust ball bearing close to the shaft shoulder of the transmission shaft; an elastic part is arranged between the first clamping plate and the second clamping plate in a clamping mode; and the elastic part is pressed by force when the nut is tightly screwed. According to the pre-tightening device of coupled thrust ball bearings, the circular inclined section spring is adopted, the contact area is larger and elastic force is uniformed. During arranging, the spring is forced togenerate certain pre-compression by screwing the nut tightly in different degrees, and amount of clearance generated by abrasion of a rolling element and a roller path can be compensated automaticallyin virtue of a characteristic of automatic recovery of deformation of a spring.