When the number and structure of transmitting antennas are fixed and transmit beampattern requires a wider 3 dB mainlobe width, the mainlobe of transmit beampattern will be split in most of the transmit beampattern optimization design methods. Aim at this, a low sidelobe transmit beampattern optimization design method for MIMO radar with mainlobe shape-preserving is proposed in the paper. The proposed method can effectively avoid the phenomenon of mainlobe splitting by guaranteeing that the line segment between any two points in mainlobe region is always inside the mainlobe region of transmit beampattern, and obtain the low sidelobe level by using a modified matrix whose diagonal elements are 0 to modify the non-diagonal elements of transmit waveform covariance matrix. The transmit waveform covariance matrix and the modified matrix can be simultaneously optimized by constructing optimization model. Simulation results show that the transmit beampattern synthesized by the proposed method not only has the desired mainlobe shape, but also has lower sidelobe level.