A clamping device configured to automatically pull and clamp workpieces includes a clamping module and a driving module. The clamping module includes a sliding block, two clamping members positioned at two sides of the sliding block, and a connecting plate connected to one end of the sliding block. The driving module includes a driving member. The sliding block defines at least one pair of guiding grooves, and a distance between the paired guiding grooves is increased in a direction toward the connecting plate. The two clamping members are mounted on the guiding grooves of the sliding block in a sliding way. The connecting plate is fixedly connected to the driving member. When the connecting plate and the sliding plate are driven by the driving member, the two clamping members move toward or away from each other to clamp or release the workpiece.