The work aims to analyze the rocking characteristics of floating transport cargoes under vehicle impact load and evaluate their capsizing risks. The floating cargo or package was simplified into a rigid mass block, and the carrying vehicle was modeled as a rigid support with a certain horizontal acceleration load. The governing equation of the rocking motion of the rigid mass block under the acceleration load was derived. The impact load waveform of the vehicle under the deceleration braking condition was measured through the test design, and the mathematical expression of the impact load wave was established. The dynamic rocking characteristics and capsizing risks of rigid mass blocks under impact loads were analyzed by the theoretical analysis method in the approximate linearization range. When the rigid mass entered the rocking response under the forward acceleration load, the initial rocking angle was less than 0 and it was capsized in the free vibration state. The minimum capsizing acceleration spectrum of the rigid mass block under the impact load of half sine wave was plotted. There was a boundary line to divide the spectrum into the safe zone and the capsizing zone. The aspect ratio of the rigid mass block was positively correlated with its stability. The rocking characteristics and capsizing risk of floating cargoes are studied by linear approximation of the rocking response system problem transformed into a rigid mass block.