Sampled fiber Bragg gratings (FBG) have many applications in wavelength division multiplexed (WDM) systems, and can be filtering or chromatic dispersion compensating simultaneously in multiple channels. Periodic phase-only sampling is a preferable approach to the design of multi-channel FBG, which facilitates the fabrication of multi-channel FBG. However the periodic sampling function replicates a given chromatic dispersion function, as a result, all the channels are identical and the resulting device cannot compensate for the dispersion slope. An optimization method is presented to design weakly aperiodic phase-only sampled gratings, and a set of chirp controlling parameters, which are related to chromatic dispersion in each channel, are introduced to compensate the chromatic dispersion and the dispersion slope simultaneously. By using the simulated annealing method, an optimized set of the relative channel phases can be found to reducing the difference between the minimum and the maximum values of the amplitude modulation of the sampling function. Then, an iterative procedure, called the Gerchberg-Saxton algorithm, is applied to remove the amplitude modulation of the sampling function, and the aperiodic phase-only sampled grating is synthesized successfully. Finally, the complex reflection spectrum of the aperiodic phase-only sampled grating is simulated by using the transfer matrix method. All time delay curves in all channels are overlapped in one figure for a visible comparison of their slope, and the result shows that the dispersion compensation capacity in each channel can match with the dispersion slope of fiber link exactly. However, as a price of the Gerchberg-Saxton algorithm, the additional channels appear at both sides of the reflection spectrum.