The invention provides a dipping small-object detecting sonar system and a detecting method thereof. The dipping small-object detecting sonar system is composed of an underwater part and an on-water part, wherein connection and dipping between the underwater part and the on-water part are realized through an armored photoelectric composite cable. The underwater part comprises a transmission energy transducer, a double-layer cylindrical receiving array, an underwater signal processor and a serial server. The on-water part is a display console and performs data exchange with the underwater part through a water-surface optical terminal. After related parameters of the underwater signal processor and a transmission control panel are configured through the upper display console, the underwater part is dipped to an appropriate depth. The signal processor processes data which are output from the receiving array; the signal processor performs operations of wave beam formation, target detection, target positioning, etc.; and furthermore the signal processor uploads data of detection result, array gesture, depth, etc. to the upper display console for real-time displaying, thereby realizing 360-degree panoramic detection. The dipping small-object detecting sonar system and the detecting method thereof are used for various operation platforms and have advantages of low cost, small size, small weight, flexible configuration and high detecting performance.