The invention discloses a terahertz isolator device of a magnetic surface plasma waveguide and a control method thereof. The asymmetric and periodic surface plasma waveguide is formed by a metal wall and a semiconductor indium antimonide pillar array. By exerting an external magnetic field under low temperature, the indium antimonide shows the revolving electrical property, and the structure can generate a magnetic surface plasma mode, so that the function of unidirectional isolation transmission of the terahertz wave can be achieved. The unidirectional transmission working frequency range is higher than 1THz, the bandwidth is larger than 80 GHz, the isolation reaches 90dB, the insertion loss is lower than 0.25dB, and tuning on the working frequency range can be performed by controlling the external magnetic field strength. Under the temperature of 186K, the frequency of the external magnetic field can be adjusted to 0.7T from 0.1T, and the central working frequency of a unidirectional transmission frequency band of the device can be tuned to 1THz from 1.42THz. The terahertz isolator is low in loss, high in isolation, tunable in broadband and capable of reducing echo and scattered noises in a terahertz application system and improving transmission quality of the terahertz beam.