The smoothness of the optical field on target surface in the laser inertial confinement fusion was studied based on fiber amplification network. The lens array was used to smooth the optical field generated by multi-beam multi-mode fiber laser after beam combining. The corresponding experiment was carried out with proper lens array. As a result, a small focused light spot was obtained on the target surface, with RMS less than 5% on the top region of the optical field. The influence of the target surface position and number of beam combining lasers on the smoothness of final optical field was also studied in the experiment. Proper target surface position and the increased number of beam combining lasers obviously improved the smoothness.