Abstract: Both of cavitation erosion and air entrainment against cavitation are important aspects of studying high-speed jets. Under the condition of slight submerged outlet and constant ratio of chamber area to outlet of jet, decompression tests are carried to study cavitation characteristics of round jet in stilling basin with sudden enlargement and abrupt drop. In which, the pressured rectangular chamber with gate slots followed by round pipe flow is located before the stilling basin. In experiments, two offset height S (5.5 m and 4.0 m) and two round pipe length L₂(L₂/d=2, 1, 0, d is round pipe diameter) are tested. The experiments reveal that the probability of cavitation on gate slots and sidewalls of stilling basin reduces by 10% when the offset height decreases from 5.5 m to 4.0 m. Moreover, the probability of cavitation on gate slots with L₂/d=2 and 0 increases by 40%-60% compared with layout of L₂/d=1, while for sidewall in stilling basin the probability of cavitation with L₂/d=0 is higher 260% compared with layout of L₂/d=1. But its adverse effect on cavitation becomes smaller as L₂ continues to increase. Furthermore, the offset height should not be too low in order to keep flows with enough aeration. Therefore, the effect of offset height S on flow cavitation of gate slot and sidewall in stilling basin is small while the effect of length of round pipe is relatively greater.