Terahertz metamaterial phased array antenna has significant application value in integrated detection and communication systems due to its strong beam manipulation capabilities. In order to enhance the communication and detection performance of the system, it is necessary for terahertz metamaterial phased array antenna to achieve flexible switching between wide beams and narrow beams. Therefore, a beamwidth control method of terahertz metamaterial phased array antenna based on inverse phase interval(inverse phase) coding is proposed in this paper. The array size is reversed by the antenna 3dB beamwidth expression and the target beamwidth, the phase encoding is kept unchanged while the remaining elements outside the array size are inversely encoded, aiming to achieve phase cancellation by phase inversion. The simulation results demonstrate that the proposed method accurately controls the beamwidth of terahertz metamaterial phased array antenna, allowing for flexible switching between wide beams and narrow beams.