It is necessary to analyze the power spectral density of modulation mode in optical communications. A detailed derivation of power spectral density of the dual amplitude pulse positiion modulation(DAPPM) scheme is presented and the continuous spectrum and discrete spectrum of DAPPM spectrum are analyzed respectively. Numerical results show that the spectral profile resembles a general sinc envelope shape, which contains direct component. The amplitude coefficient α has nothing to do with shape of continuous spectrum, but the amplitude of continuous spectrum. The amplitude of both components and the location of the slot frequency component largely depend on the modulation level and the pulse duty cycle. The maximum power level of slot frequency components is achieved when pulse duty cycle is 0.5, and decreases linearly at a rate of about 5 dB with the increase of modulation level. It is also shown that the slot frequency is hard to pickup from random DAPPM sequence at large modulation level.