The invention discloses an apparatus for testing the radial deformation of an inner pore of a built-in rock sample. The apparatus includes a ring-shaped extensometer which may be placed inside the inner pore of a to-be-tested rock sample and may deform along with the radial deformation of the inner pore of the to-be-tested rock sample, an attaching apparatus which is intended for attaching the ring-shaped extensometer to the wall of the inner pore of the to-be-tested rock sample and a sensor which is intended for sensing the deformation of the ring-shaped extensometer. The ring-shaped extensometer is a circle ring having an opening. The opening of the circle has one end provided with a sliding pole which may freely slide relative to the other end of the opening in the radial direction of the sliding pole. According to the invention, the apparatus may be placed inside the inner pore of the to-be-tested rock sample, and is provided with the attaching apparatus, such that the extensometer is in tight contact with the internal wall of the inner pore of the to-be-tested rock sample. Therefore, the apparatus makes it easy to measure radial deformation of the inner pore of the rock sample and has precise measurement.