A device and a method for testing a compression amount of a pile body of a rock-socketed cast-in-place pile is provided. The testing device includes open flexible pipes which are correspondingly bound with two main reinforcements in the pile body of the rock-socketed cast-in-place pile, and lengths of the open flexible pipes are the same as those of the bound main reinforcements. One end of each of the two open flexible pipes is located at a bottom end of a corresponding main reinforcement and fixedly connected with a first sealing sheet, and other ends of the two open flexible pipes are located at a top portion of the rock-socketed cast-in-place pile and fixedly connected with second sealing sheets. A closed rigid pipe is located in the open flexible pipe, and pipe bodies of the closed rigid pipe and the open flexible pipe are not in contact.