Acoustic field characteristics of an ultrasonic linear phased array are studied under the condition of an interface. The issue about a phased array transducer with a wedge is reasonably simplified into a liquid-solid interface problem. According to ray acoustics theory, the acoustic field produced by a single element on the liquid-solid interface is computed. Then focus laws are derived and the expressions of the sound field and the displacement of the ultrasonic linear array with a liquid-solid interface are obtained. Simulations are carried out for a phased array transducer mounted on a wedge. The focusing-generated effects on axial and horizontal acoustic field of transducer are discussed. It is shown that the focusing can enhance sensitivity and resolution, but degrades the performance of beams beyond the focal region. So focusing should be reasonably used in actual testing.