An apparatus for reinforcing a jet pump riser includes: an elbow upper clamp for covering a riser elbow coupled to a thermal sleeve from an upper side thereof; an elbow lower clamping member for clamping the riser elbow from a lower side thereof; an elbow vertical portion clamping member for covering a vertical portion of the riser elbow; and an elbow horizontal portion clamping member for covering a horizontal portion of the riser elbow. These members are disposed in different orientations with respect to the elbow upper clamp so as to fix the thermal sleeve, the riser elbow and the riser pipe.