The invention, which belongs to the technical field of underwater detection, provides an underwater ultrasonic phased array detection system and an underwater detection method. The underwater ultrasonic phased array detection system is composed of an ultrasonic phased array transducer, a receiving terminal and a processor. Phase excitation sequences and delays of transmitted signals of all array elements are controlled and ultrasonic beams with continuously changing directions are synthesized to scan underwater different directions; and all array elements are able to receive reflected waves and send the reflected waves to the receiving terminal; the receiving terminal converts the reflected wave into echo signals and sends the echo signals to the processor; and the processor carries out excitation sequence and delay controlling on the array element transmitted signals and calculate target position information based on the echo signals provided by the receiving terminal. Therefore, detection of random underwater directions is realized; and the scanning range of the target is extended. Besides, the underwater detection method is a using method based on the underwater ultrasonic phased array detection system.