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Giant magnetostrictive material (GMM) has the smart potential to be integrated as a self-sensing actuator. This paper presents a novel self-sensing giant magnetostrictive actuator (SSGMA), by sensing the on-line stiffness of the actuator upon the ΔE effect. A self-sensing signal is generated by superimposing a set of high-frequency small sensing excitation magnetic fields on low-frequency static or quasi-static driving magnetic fields. The fully coupled magneto-elastic-thermal nonlinear constitutive model of GMM is derived, and then the self-sensing response model of the SSGMA based on the nonlinear equivalent piezomagnetic equation is proposed. On the theoretical basis, the influences of magnetic field, prestress and temperature on the ΔE effect, the equivalent piezomagnetic equation parameters and the SSGMA sensing signal are investigated in detail, respectively. Moreover, a prototype of the SSGMA is fabricated and tested for self-sensing performance. The experimental results demonstrate the effectiveness of the theoretical analysis, and further show that the proposed SSGMA achieves self-sensing output displacement within a stroke of nearly 50 μm, with a sensitivity of 2.49 mV/μm. The self-sensing displacement resolution of the SSGMA by far may reach 63.4 nm after experimental determination. This novel self-sensing actuator with micron-level self-sensing drive capability can be integrated into an external sensorless execution system in the future.