The invention discloses a rock core fissure measuring instrument and a rock core fissure measuring method. The rock core fissure measuring instrument comprises a strike angle measuring scale and a dip angle measuring scale which are combined for use, wherein the strike angle measuring scale is of an annular structure, and 0-360 degrees scale lines are arranged on the edge of the annulus; the dip angle measuring scale is of a transparent disc-shaped structure, and four 0-90 degrees' scale lines are uniformly distributed on the edge of the disc; a rotatable pointer for indicating the dip angle of a fissure is mounted at a central position of the dip angle measuring scale; a poking handle is arranged on the pointer; the dip angle measuring scale is mounted at the 0-degee scale line of the strike angle measuring scale; the planes, to which the dip angle measuring scale and the strike angle measuring scale belong, are perpendicular to each other; the center of the dip angle measuring scale is superposed with the 0-degee scale line of the strike angle measuring scale; the 0-degee scale line of the dip angle measuring scale is parallel to the plane to which the strike angle measuring scale belongs. Through the technical scheme provided by the invention, the rock core fissure occurrence parameter measuring difficulty is overcome; the rock core fissure measuring instrument has the advantages of being low in cost, simple to operate, high in measuring precision and the like.