A deformation testing device includes a base holding a workpiece and a testing machine arranged on the base. The testing machine includes a distance-measurement device and a force-applying device. The distance-measurement device includes a distance meter metering a moving distance of an elastic portion of the workpiece and includes a measurement probe. The force-applying device includes a force meter and a transmission member. The force meter slidably is mounted on the base and includes a force-applying post. The transmission member includes a first post and a second post secured to the first post. The first post is aligned with the force-applying post transmits a force from the force-applying post to the elastic portion. The measurement probe abuts against the second post. The force meter applies and measures a force acted upon the elastic portion through the force-applying post and the first post.