Herein, we propose a scheme for developing a large-momentum-transfer atom interferometer based on the top-hat composite light pulse technique. Additionally, we analyze the contrast and phase noise using a theoretical model of the sensitivity function of the proposed atom interferometer. A top-hat composite light pulse is used to simulate calculations based on the atom interferometer. We confirm that compared with a Gaussian beam, a top-hat composite light pulse can improve the consistency of atom cloud transitions and increase the contrast of atom interference fringes. By designing symmetrical and reversed composite pulse sequences, the phase noise and vibration noise in the time interval and free evolution process of multipulse action can be suppressed. The numerical simulation results show that the sensitivity of the proposed atom interferometer using a top-hat composite light pulse increases by one order of magnitude compared with that using a Gaussian beam. Moreover, the proposed atom interferometer achieves satisfactory suppression of external technical noise.