For ship target detection applications under complex sea conditions, the ship/sea contrast of traditional single-band spaceborne Synthetic Aperture Radar (SAR) is low, and the ship target detection performance decreases sharply. In this paper, a concept of spaceborne tri-band common caliber SAR system is proposed. The antenna technology of tri-band common caliber SAR is adopted to reduce the antenna area and weight, and the electromagnetic scattering information of L, S and X bands can be obtained simultaneously. This paper introduces the tri-band SAR echo information joint processing method. Firstly, the joint processing from the perspective of SAR signal is carried out, and quickly detect the ship target location, then small scene imaging of target area is obtained. After that, through the tri-band SAR image fusion processing, further enhance the Signal-Clutter-Noise Ratio (SCNR) and outline of the ship target information, and improve the ship target detection performance of complex sea condition.