According to the digital spiral imaging method, the information of the object is recorded by the orbital angular momentum of light. Through analyzing the orbital angular momentum spectra, we obtain the information of the phase object. Characteristics of the orbital angular momentum spectra of a simple canonical phase object are studied when different Laguere-Gaussian (LG) beams are used in detection. The results show that we can obtain the phase and transmittance information effectively by analyzing the orbital angular momentum spectra. Spectral components with different diffraction orders produced by the LG beams with different topological charges and radial node numbers are analyzed. The results show that the high-order LG beam is more conductive to probe the information of a simple canonical phase object.