In order to design and evaluate the fuze's inertial forward firing mechanism of a medium caliber naval shell when shooting at sea, with the help of the commercial software ANSYS/LS-DYNA, using the arbitrary Lagrangian-Eulerian method, the water entry process of projectile with different falling speed and different falling angles was simulated. The water entry set forward overload coefficient of the projectile under the conditions of different falling speed, angle and rotation were got. The greater the falling speed or the falling angle, the greater the set forward overload, the smaller the overload peak width. The attitude of projectile was unstable when it fell into the water with small angle, and ricochet easily occurred. The set forward overload of projectile was reduced or even disappeared, and larger radial overload was produced. Rotation of the projectile entering the water had little effect on set forward overload and it could be neglected. When the overload of fall velocity was 250 m/s, and the falling angle was 5 degrees, the confidence limiting ballistic environment for inertial firing mechanism of fuze which firing accuracy design must be considered, the peak value of set forward overload was about 287 g, and the duration was about 1 ms.