Water content in crude oil is very important in an oilfield production logging system. A non-contact optical method, called all-optical detection, was proposed for measuring the water content in the full range of 0-100% in crude oil by a non-contact laser source and receiver (adaptive laser interferometer). The laser-induced ultrasonic P-wave velocities of three series of an oil-water mixture accelerated significantly from 1289, 1342, and 1343 m/s, respectively, to 1458 m/s with the increase of the water content from 0 to 100%. The theoretical values of the effective medium model were agreement with the experimental results of three series of an oil water mixture.