The ratio between the intensity of focused spot and the total intensity of the fiber output, which is spot focusing efficiency, affects the sampling signal to noise ratio of single-fiber digital scanning based on liquid crystal spatial light modulator (SLM). Using sequence coordinate ascend algorithm to control the input field, the light emitted from the multimode fiber can be focused. The theoretical and experimental results show that the sum of normalized objective function and spot focusing efficiency is approximately constant. In order to improve spot focusing efficiency, the normalized objective function value must be minimized. The affect of the multimode fiber output light intensity on the convergence of the normalized objective function is analyzed. The numerical simulations and experiments show that the normalized objective function convergence rate changes slowly and spot focusing efficiency deteriorates with the output light intensity increasing.