A magnetostrictive force sensor that can be used to measure alternating load is presented. Iron–gallium alloy (Galfenol) is used as the sensing element. The sensor was developed based on a closed structure. A finite element method was employed to optimally design the size of the excitation coil and the flux path. Simulation results show that the flux density distribution can be clearly improved after optimization. Energy efficiency analysis shows that the energy efficiency of the proposed sensor can be increased more than 5 times that of the open structure sensor. To study the sensing behavior, a discrete energy–averaged model was developed to analyze the susceptibility change of Galfenol when it was loaded with alternating stress. Experimental results show that the model can predict the change in susceptibility with good accuracy. The linear part of susceptibility was selected to calibrate the sensor with an alternating load of ±1270 N magnitude. Experiments were conducted to verify the calibration.