An illuminating lens design method and an illuminating lens. The projections of the highest points of an outer surface and an inner cavity surface of the illuminating lens on the horizontal plane are overlapped, and a horizontal profile curve of the outer surface and the inner cavity surface of the illuminating lens obtained from an arbitrary height satisfies: ( | x i | a i ) n i + ( | y i | b i ) n i = 1 where ai is the projection length of a portion of an xz section basic profile curve, which is intercepted by an xiyi plane, on the xiyi plane; bi is the projection length of a portion of a yz basic profile curve, which is intercepted by the xiyi plane, on the xiyi plane; xi, yi are coordinates of certain point on the outer horizontal profile curve and the inner horizontal profile curve; ni is a real number greater than 1.