For terahertz systems where reflected signals carry effective information, such as terahertz time-domain reflection systems and full-duplex communication systems, existing nonreciprocal terahertz devices often treat reflected signals as interference and suppress them during isolation. This makes them incompatible with the requirements of such systems for isolating incident signals while directionally extracting and detecting reflected signals. To address this limitation, this study innovatively proposes a terahertz isolator based on a magneto-optical selection–multiport architecture. The device converts linearly polarized light into a specific circular polarization state through orthogonal double gratings and, combined with the magneto-optical selectivity of InSb material, constructs a nonreciprocal transmission path. Furthermore, the magneto-optical regulation mechanism innovatively incorporates branch waveguides with multiple ports and the characteristic of regulating terahertz transmission paths, simultaneously achieving isolation of incident/reflected signals and directional extraction of reflected signals. By simulating the influence of structural dimensions and external environmental conditions on the nonreciprocal characteristics of the device, it is found that under a temperature of 250 K and a magnetic field of 0.3 T, with the structural parameters set as branch length of 170 μm, center-to-center spacings of adjacent branches of 125 μm, 125 μm, 120 μm, and 120 μm, InSb layer thickness of 5 μm, grating layer thickness of 50 μm, and substrate layer thickness of 20 μm, the device achieves a high isolation of 63.12 dB at 0.73 THz. Additionally, at 0.78 THz, the bidirectional transmission efficiency reaches 36.31%, with a 3 dB bandwidth of 0.25 THz. This device offers advantages such as high isolation, low operating magnetic field strength, and integration of dual functions. It reduces interference from incident signals on reflected signals, simplifies subsequent processing steps such as noise reduction and localization of effective reflected signals, and enhances the system's detection performance for weak signals. This provides essential support for expanding terahertz applications to more fields, including non-destructive testing and communication.