The performance of MIMO radar is directly affected by its transmitting waveforms. The waveform design is one of the critical issues in the design of MIMO radar system. In this paper, a new MIMO radar waveform design method based on time domain and frequency domain joint optimization is proposed. Firstly, a continuous phase coded signal waveform set is chosen to be the optimization objective variable. The design objectives is that all waveforms in the set are orthogonal in time domain, and the power spectral density (PSD) of every waveform approximates to the desired distribution in the frequency. According to the requirements, the problems in time domain and frequency domain are analyzed, respectively. Meanwhile, two objective functions based on minimizing the weighted correlation sidelobe level (MWISL) and minimizing the stopband power spectral density (MSPSD) are introduced. Then, an optimal scale factor is introduced to integrate the time domain and frequency domain into a combined model and a close form solution of code element is deduced through proper simplification and equalization of the time-frequency (T-F) joint optimization model. A recursive algorithm is obtained by summarizing the derivation process. At last, numerical examples prove the effectiveness of the proposed method in matching desired spectrum distribution and decreasing correlation sidelobe.