To get the laser acoustic energy conversion efficiency, TEA-CO2 pulse laser, a spherical piezoelectric ceramic hydrophone and a digital oscilloscope were used to receive, demonstrate and store the optoacoustic signal induced in water. And energy conversion efficiency η was calculated. The results showed that when the laser pulse energy varied below 2 J/cm2, the conversion efficiency η was in the range 10-7～10-5, and increased ten times when the laser beam was focused. If the spectrum width of the laser pulse was fixed, η increased with the pulse energy, on the other hand if the pulse energy was fixed, η decreased when the spectrum width increased. As a result, methods of laser focusing, laser pulse compression and laser energy increasing can be used to enhance the conversion efficiency η, and to get strong thermoelastic optoacoustic signal.