To the characteristics of large error in sea target positioning, the method of Multiple Rounds Simultaneous Impact (MRSI) of guided projectile can effectively improve the hitting accuracy and firepower density. The shipborne gun with controllable muzzle velocity can adjust the muzzle velocity of guided projectiles, correct the trajectory of the projectile, and attack the target by using the preplanned guidance law. The models of projectile exterior ballistic and the optimum guidance law with angle constraints are established, and the method for calculating the predicted point of sea target is proposed. According to the guided projectile trajectory characteristics, an analysis is made to the fire control method of the shipborne gun with fixed muzzle velocity, based on which the fire control method of the shipborne gun with controllable muzzle velocity is studied. The simulation results show that the controllable muzzle velocity shipborne gun can greatly increase the number of simultaneous projectiles, and enhance the firepower density.