Aiming at solving the low detection probability problem of space-based AIS, the received signal collision phenomenon and ship detection probability model were studied. The difference between Space-based AIS and traditional terrestrial AIS was compared and analyzed. And the new technical challenge of space-based AIS was pointed out. The problem of AIS signals collision from multi SOTDMA cells was studied in detail. The detection probability model based on ship density function was established theoretically. The effects of slot collision, orbit height, antenna swath and Class B ships on detection probability were estimated quantitatively by simulations, which can provide theoretical reference for designing and evaluating the space based AIS in the future.