Abstract: The Mosaic mode is a hybrid mode of spotlight and ScanSAR, which can obtain SAR images with a large coverage and a high resolution. In this paper, a new Mosaic mode is proposed. The image scene is divided into several sub imaging blocks along the range and azimuth direction. Each block is imaged by sliding spotlight and then pieced together. Short transmit sub-aperture makes the scanning angle decrease significantly, which reduces the difficulty of system design. It also makes the range migration of each block smaller. Azimuth multichannel is helpful to improve the system Signal to Noise Ratio (SNR). PRF (Pulse Repetition Frequency) can be reduced by space sampling, which is constructive to select the proper PRF in the timing diagram.