With the continuous upgrading of vehicle diesel engine emission regulation and power density, the turbocharger rotating speed and turbine inlet temperature also increase continuously and hence the risk of thermalmechanical fatigue failure of the turbine wheel backdisc increases significantly. In order to improve the reliability of turbine backdisc, a backdisc cooling technology for radial turbine was proposed. The influences of the relative mass flow rate of cooling air introduced from the pipe after an intercooler on the thermal stress of turbine backdisc and the performance of turbine were studied by the fluidsolid coupling numerical simulation. The results show that the thermal stress of backdisc decrease by 31 MPa, 108 MPa and 132 MPa respectively and the synthetic stresses reduce by 3.8%, 13.1% and 16.0% respectively when 1.0%, 2.0% and 3.0% relative cooling flow is consumed. The effect of backdisc cooling on turbine performance becomes weak. Besides, the effect of cooling on expansion ratio can be ignored and its thermal efficiency reduction is no more than 1.0% at the relative cooling flow of less than 3.0%.