The most existing MIMO radar waveform optimization designs are a narrowband signal designed according to the need of radar tasks in tracking and detecting without the requirement of imaging tasks. A kind of method about MIMO radar waveform optimization design for multi-target ISAR imaging is proposed in the paper. The prior information like target location, RCS, target velocity as well as bandwidth limitation of imaging task is considered in waveform optimization as important constraints, a waveform optimization model for multitarget Imaging is established and solved by the conjugate gradient method. The designed waveform can achieve the imaging of targets in different directions at the same time.