In view of the problem that the superstructure vibration of a feeder container vessel exceeds the allowable standard during the sea trial, the on-site analysis of the reasons for the vibration of the ship superstructure was carried out at sea based on the vibration test results of the real ship and the vibration calculation report of the finite element numerical simulation. Meanwhile, a practical structural strengthening plan is proposed to strengthen the pillars of the superstructure directly at sea according to the structural form of the vessel, the internal outfitting, and the layout of equipment such as material cranes and lifeboats. After strengthening, the vibration results of each test position of the superstructure can meet the requirements of the specifications. This not only avoids the high cost of the second sea trial, but also provides the necessary conditions for the smooth delivery of the vessel.