Terahertz (THz) science and technology promise unique applications in high-speed communications, high-accuracy imaging, and so on. To keep up with the demand for THz systems, THz dynamic devices should feature large phase shift modulation and high speed. To date, however, only a few devices can efficiently manipulate the phase of THz waves. In this paper, we demonstrate that efficient phase modulation of THz waves can be addressed by an active and enhanced resonant metamaterial embedded with a nanostructured 2D electron gas (2DEG) layer of a GaN high electron mobility transistor (HEMT). The enhanced resonant metaunit couples the traditional dipolar and inductance-capacitance resonances together to realize a coupling mode with enhanced resonance. Embedded with the nanostructured 2DEG layer of GaN HEMT, the resonance intensity and surface current circuit of the enhanced resonant mode in the metamaterial unit can be dynamically manipulated by the electrical control of the carrier distribution and depletion of the 3 nm 2DEG, leading to a phase shift greater than 150° in simulation. In the dynamic experiments, a 137° phase shift was achieved with an external controlling voltage of only several volts in the THz transmission mode. This work represents the first realization of a phase shift greater than 100° in a dynamic experiment in transmission mode using an active metamaterial structure with only a single layer. In addition, given the high-speed modulation ability of the HEMT, this concept provides a promising approach for the development of a fast and effective phase modulator in THz application systems.