Blue-green laser has an important application prospect in the fine detection of underwater targets. Based on the fast numerical traversal algorithm of the head-tail circumferences, the laser scanning detection process of underwater targets was numerically modeled by establishing the intersection equation of the scanning beam and the moving target. The results show that the shortest time for the target to cross the beam scanning area does not occur in the case of the head-on encounters. Secondly, by Monte Carlo simulation, the effects of pulse frequency, scanning frequency, scanning half angle and target distance on the detection probability were simulated and the optimal design of scanning parameters was obtained. The established numerical model can provide a theoretical basis for the reasonable design of underwater laser detection system in this paper.