A clamping device includes a fixing clamp and a positioning clamp for clamping two plate pieces. The positioning clamp includes a main body having a first abutment surface and a second abutment surface separated by an abutment angle and suitable for respectively abutting against first plate surfaces of the plate pieces. A reversing mechanism includes a reversing member rotatably disposed in the main body and having a limiting portion. The reversing member is rotatable relative to the main body between a first side and a second side, in which the limiting portion is aligned with the first abutment surface and the second abutment surface, respectively, and is suitable for abutting against the end surfaces of the plate pieces.