The invention provides an underwater robot object detection device and a detection method. The device comprises a PC/104 computer, a single-beam forward looking sonar equipment sensor, a Doppler log, a depth gauge, an altimeter and a relay element. When in an underwater state, after the computer is electrified, an equipment control program is started automatically, controls a relay to open the sensor through a DI/O digital board, collects the environment information in the advancing direction with a single-beam forward looking sonar, calibrates the data with the information of the Doppler log, classifies the data through the depth information, obtains underwater object point plane information through processing algorithm, and estimates the object underwater depth information based on the information of the depth gauge, so as to detect an underwater object. The invention prevents the space occupation problem of the multi-beam forward looking sonar and the three-dimensional sonar, effectively solves the problem that the single-beam forward looking sonar can only obtain the underwater object two-dimensional detection information, and is more applicable to the underwater object detection application of an underwater unmanned submarine.