The problem of designing a unimodular waveform with the similarity constraint for the colocated multiple-input multiple-output radar is investigated. A new approach named by the phase-based multiplier method is proposed to optimise the waveform subject to both the constant modular constraint and the similarity constraint to maximise the signal-to-interference-plus-noise ratio in the presence of the signal-dependent interference and the additive Gaussian white noise. Compared with the existing alternative methods, the new approach has lower computational complexity. Furthermore, the numerical simulations demonstrate that the performance of the proposed algorithm outperforms the existing methods.