Abstract: Outstanding beamforming performance of the Multiple-Input Multiple-Output (MIMO) radar can be achieved by deploying a large number of active antenna elements. Nonetheless, this will significantly increase power consumption, circuit complexity and hardware cost. These problems can be overcome by utilizing low-resolution Digital-to-Analog Converter (DAC) components. However, MIMO radar waveforms designed under the condition of infinite-resolution DACs are usually inapplicable to systems with low-resolution DACs. Therefore, under the constraints of discrete phases, this paper proposes a MIMO radar constant modulus waveform design method based on Integrated Sidelobe-to-Mainlobe Ratio (ISMR) minimization. The Dinkelbach algorithm is first used to convert the objective function with quadratic fractional form into a subtraction form. Then, the alternating direction penalty method is employed to solve the nonconvex constant modulus discrete phase constraint problem. Finally, by comparison with other methods through numerical simulations, the behavior of the transmit beampattern and the performance of ISMR are analyzed, and the effectiveness of the method is verified.