Abstract: Considering that there is no strong theory support to the design of hydraulic parameters in conductor jetting in deep water drilling, the calculation formulas of critical jet outlet velocity and flow rate in jetting were derived based on the water jetting theory and rock mechanics, by which an optimal design methods, criteria and procedures were presented, and investigated. The combination patterns of different bits and conductors used in jetting, showing that the critical jet flow rate was 69.5 L/s for the combination of φ660.4 mm bit and φ762.0 mm conductor, with 24.0 mm equivalent nozzle diameter, and the maximum breaking diameter was 762.0 mm when the nozzle diameter was 26.0-30.0 mm. Given constant jet flow rate, the smaller the equivalent nozzle diameter, the greater the breaking strength is. Taking the design of hydraulic parameters in deep water jetting drilling in JDZ Block, West Africa, as example, the error between designed and actual jet flow rates was less than 10% if φ14.3 mm nozzle was used. It proved that this optimal design method was reasonable and it could be effectively used in design of hydraulic parameters in deepwater conductor jetting operations.