The invention relates to a device for measuring the convex value of a bearing preloaded with less than or equal to 10 neutons, wherein a pedestal 6 is arranged on a platform 7.1 of a magnetic movement rest 7; a guide sleeve 4 is fastened on the pedestal by a bolt 5; a reference block 8 or a bearing to be measured 3 can be arranged inside the guide sleeve; a load block 2 is arranged on the reference block 8 or the bearing to be measured; a measuring head of a dial gage 1 which is arranged on a supporting arm 7.2 of the magnetic movement rest 7 is pointed at a center hole of the load block; theoverall dimension of the reference block is the same as that of the bearing to be measured; the weight of the load block is consistent with the designed preload of the bearing to be measured; and theoutside diameter of the reference block or the outside diameter of the bearing to be measured is slidably matched with the inside diameter of the guide sleeve. The device does not require any power source such as power, gas and so on, directly adds load blocks with different weights on the bearing, takes the weight of the load blocks as the load, and can measure the convex value of the bearing through comparison measurement with the reference block.