Phase shifters are important devices in phased array systems. As the frequency increases, the skinning depth of the metal and the roughness of the waveguide surface have a non-negligible effect on the device, which cause the increase o f device loss. To address the difficulties of poor phase shift accuracy and high loss in terahertz phase shifters, a planar digital terahertz phase shifter based on a loaded line is proposed. By connecting two branches in parallel on a microstrip line, the switching diodes are loaded on the parallel branches, the electrical lengths of the two branches are adjusted to obtain the desired phase shift, and the switches states are controlled to achieve different phase shift angles. The simulation results show that the reflection coefficients of both on and off are less than -10 dB, the insertion loss is less than 0.5 dB, and the phase shifting error is less than 5 °in the 192~210 GHz band range, while the phase shifting error is less than 1 °within the 5 GHz band. The proposed planar phase shifter is easy for processing and system integration with low cost, and has a wide range of applications in terahertz phased array systems.