Hydrocyclone is a crucial device of waste water treatment in aggregate processing for hydropower projects. The separation efficiency of hydrocyclone determines the success of the whole processing system. To date,the operational parameters of hydrocyclone are empirical. To quantify the impact of inlet pressure and concentration as well as hydrocyclone parameters on separation efficiency,we carried out test on the separation efficiency of hydrocyclone with a diameter of 250 mm and 150 mm,respectively. Results demonstrated that the rising of inlet pressure cannot further obviously increase the production rate of underflow as it reached 0.1 MPa. As inlet concentration increased,the production rate of underflow decreased. Hydrocyclone with a diameter of 250 mm or 150 mm is not recommended to be used alone as the former has a production rate lower than design value,which results in unsatisfactory performance of some processing techniques,and the latter has a concentration of underflow lower than the requirement for the next dehydrate process. We recommend to use hydrocyclone with a diameter of 200 mm alone,or a diameter of 250 mm and 150 mm in parallel running. The research findings offer experience for the design and operation of waste treatment in aggregate processing.