The invention relates to a non-unit self-adjoint impedance protection method applicable to a half-wavelength power transmission line. The method comprises the following steps of acquiring a three-phase current and a three-phase voltage of a relay protection device installation side of the half-wavelength power transmission line; judging whether starting quantity of the half-wavelength power transmission line conforms to a starting condition or not; determining variable quantity impedance of the half-wavelength power transmission line; combining steady-state quantity impedance and variable quantity impedance of the half-wavelength power transmission line into self-adjoint impedance of the half-wavelength power transmission line; and opening relay protection action of the half-wavelength power transmission line according to the self-adjoint impedance of the half-wavelength power transmission line. According to the method provided by the invention, relay protection action is very rapidly and reliably carried out when a fault occurs in a circuit by means of non-unit electrical quantity of the half-wavelength power transmission line, false action of the fault outside a circuit region can be prevented, and the reliability and the sensitivity of relay protection of the half-wavelength power transmission line are improved.