In this paper, an online search method for the maximum efficiency operating point of a synchronous reluctance motor (SynRM) based on virtual signal injection is proposed. The SynRM and the inverter are modeled as a whole for their losses and the system efficiency is calculated. A unipolar square wave is injected into the current angle reference of the control system and the first-order derivative of the system efficiency is extracted as a differential criterion, which can be tracked to the maximum efficiency operating point of the SynRM as it converges to zero. The amplitude of the injected signal is adaptively adjusted based on the derivative of the system efficiency to reduce the steady-state error. This method effectively solves the influence of motor parameter changes and cross-coupling effects, in which the injection of virtual signals does not cause undesirable torque pulsations. Simultaneously, injecting an amplitude-adaptive square wave can decrease steady-state error and reduce current angle pulsation.By building the model in the simulation platform, this method is verified to have good dynamic performance and efficiency improvement.