An integrated optical system for cold ⁸⁵Rb atom interferometer is designed and implemented in this work. The system includes five fiber-connected modules, and the size of the entire optical system is only 400 mm × 483 mm × 133 mm. This integrated and modular optical system is beneficial for further engineering and miniaturization of atomic interferometers. An active temperature control method is adopted to improve the stability of the optical system, and the long-term stability of laser intensity reaches 4.3 × 10^-3@5000 s. This optical system has been successfully applied to an atom interferometer, and the interference fringes with a free evolution time of 71 ms and a contrast of 20% are obtained. The phase uncertainty of a single fringe is 22 mrad, and the corresponding gravitational measurement uncertainty is 26 μGal. The design and implementation of this optical system lays the foundation for the miniaturization and practical application of atom interferometers.