The studies carried out to appropriate slope ratio for underwater trench of immersed tunnel of Hongkong-Zhuhai-Macau Bridge. A test trench of 100-meter-long and 21-meter-width was excavated under Lingding Ocean along designed axis of immersed tunnel. The trench includes four slope ratio, such as 1:5, 1:6, 1:8 and 1:10. Two actual measurement which includes multibeam measurement and single beam double frequency measurement were used to measure water depth and section form in different time. It is recommended that slope ratio 1:6 can be adopted for underwater trench of immersed tunnel of HZM Bridge.