Phase-only spatial light modulator has been widely used in various fields for its high light efficiency and diffraction efficiency. However, the amplitude and phase of beam are required to be modulated simultaneously in some situations. To achieve complex amplitude modulation using a phase-only spatial light modulator, it is spatially varied diffraction efficiency in the first order direction by modulating the phase depth of a grating. Then, the phase modulation factor of grating is calculated using numerical inversion and construct a phase-only transfer function for complex amplitude modulation. Based on the theoretical analysis, doughnut beam is generated using a plane wave diffracted by a spiral phase plate modulated with Bessel-like amplitude based on the phase-only spatial light modulator. The experimental diffraction patterns are in good agreement with the theoretical intensity distribution.