This paper addresses itself to the problem of interaction mechanism of laser induced acoustic source under water. The main photo-acoustic mechanisms include thermal expansion, vaporization and optical breakdown as well as the photon beam pressure. We integrate these into a compound model and compare numerical calculation and simulation results with the existing experimental data. The different energy density thresholds between different mechanisms are calculated. We optimize original thermal expansion by considering various laser pulse-shapes especially Gaussian laser. When discussing vaporization, random bubbles distribution is studied instead of single bubble alone for the first time. Detection distance, pulse duration, laser energy and spot size in heating area all have effect on sound filed intensity, which are studied through this paper.