The internal flow field and loss distributions are quite complicated in the radial-inflow turbine. It is necessary to reinforce physical understandings of the relationship between the flow and loss. Inspired by the synergy principle in the convective heat transfer, the synergy applicable for the radial turbine is innovatively derived from the Navier-Stokes equations. According to the mathematical expression, the smaller the synergy angle is, the higher flow resistance and loss should be. The paper attempts to assess the validation of the synergy principle in the radial turbine based on numerical simulations firstly, then the relationship between the synergy angle and loss is analyzed in detail. It is found that the regions where high total pressure loss coefficient and high dimensionless entropy generation locate correspond to the relatively small synergy angle, which agrees well with the mathematical analysis. The relatively low streamwise synergy angle corresponds to the high-loss regions near the suction side and wake on the blade-to-blade stream surface. The relatively low spanwise and circumferential synergy angle correspond to the high-loss regions near the tip clearance and wake on the span-theta stream surface. Under off-designed conditions, the synergy principle also shows great performance as an apparent negative correlation of total pressure loss coefficient versus circumferential synergy angle could be perceived.