The shock waves caused by the high-speed water entry projectiles have significant effects on the trajectory and damage performance of under-water projectiles. In this paper, the light-gas gun based system was exploited to accelerate the flat and spherical projectiles into a water-filled vessel at different initial velocities horizontally. The impact induced underwater shock waves caused by the two typical projectiles were investigated on the characteristics of propagation and attenuation. The pressure history captured by the transducers installed with different distributions in water column was analyzed in terms of the attenuate characteristics according to the distances and angles. The results indicate that the peak pressure of initial shock waves undergoes an exponential decay and the pressure among the curves 1/x and 1/x2 according to the distance. The decay characteristics of pressure on the spherical surface comply with the sine curve. The attenuation experienced by the pressure for different experimental conditions are independent with initial velocity of projectiles.