The focusing beam is one of main characters of ultrasonic phased array which differs from the conventional ultrasonic test method. The sound field of monophyletic rectangular transducer and one dimensional linear array were studied in this paper. The effects of frequency, element numbers and space of element space on the focal spot and depth of focus were analyzed based on the fourdimensional slice figure. Furthermore, the influence on the resolution and sound pressure value were analyzed The optimized results of transducer parameters have certain reference value for optimal design of phased array probe.