AIris, a flat circular membrane in the middle layer of human eyeball, is controlled by sympathetic nerve and can automatically adjust pupil size according to light intensity to limit the amount of light entering the eyeball. This paper attempts to introduce the artificial iris diameter changing mechanism into hydrodynamic machinery, that is, to control the hydrodynamic retarder without filling fluid by changing the inner diameter of iris and changing the flow path of retarder. Through the decomposition and reconstruction of the intrinsic flow field, the flow field characteristics of the iris retarder are deeply understood, and the fast prediction of the braking torque is realized. At the same time, the close-loop controller is designed to control the iris opening that realizing the adaptive adjustment of the output torque of the retarder, thus overcoming the difficulty on-line observation of actual filling rate of oil problem and the inaccurate tracking of braking torque on traditional hydrodynamic retarder with filling rate control. Our work prove that the nonlinear controller can achieve fast and accurate torque closed-loop torque control in various braking conditions compared with the hydrodynamic rate control retarder, and the potential of iris mechanism for adaptive control of hydrodynamic retarder is verified.