To measure the arbitrary spectrum of orbital angular momentum (OAM) of photons, an apparatus combined a Mach-Zehnder interferometer with a pair of rotated Dove prisms to measure the arbitrary spectrum of orbital angular momentum (OAM) of photons was established. The validity of the set up is verified by comparing the measurement result with a theoretical calculation of the OAM spectrum of an elliptical beam produced by a pair of cylindrical lenses.