The multiplicity of solution in image motion estimation method based on images captured by overlapping time delay integration (TDI) imaging sensors is addressed, and a method to eliminate the multiplicity of solution by taking the optimum of smoothness evaluation function as the only solution of image motion is proposed. The existence of the multiplicity of solution is analyzed. An evaluation function is proposed to evaluate the smoothness of each solution, and the minimized evaluation function is taken as the optimal solution of image motion. The optimum is found by using the conjugate gradient method (CGM). A single-solution result is obtained with an error of 0.13 pixel, showing that the added smoothness evaluation function is feasible to solve the problem of multiplicity of solution in image motion measurement.