As an important device for the application of terahertz technology, the terahertz modulator has a wide application prospect in the fields of terahertz communication, imaging, and sensing. However, current terahertz modulators exhibit problems, such as low modulation depth, narrow operating bandwidth, and poor stability. This restricts further promotion and development of terahertz technology. In this study, a new optical control GaAs/side-polished terahertz fiber (SPTF) modulator is demonstrated. GaAs film is transferred to the polished region of the terahertz fiber to enhance the interaction with the terahertz evanescent wave. The highest modulation depth of GaAs/ SPTF modulator is 97.4% with an external laser irradiation of 808 nm. The experimental results show that this new fiber modulator exhibits a good optical control modulation effect. Additionally, the compact and highly integrated design of the device suggests its broad application potential.