This paper addresses the issue on airborne image positioning model and its application in FY-3A experiment. First, the FY-3A Medium Resolution Spectral Imager (MERSI)'s viewing vector is derived from MERSI's imaging pattern. Then, the image positioning model is analyzed mathematically in detail which is based on Earth-aircraft geometry. The model parameters are mainly determined by both the sensor - aircraft alignment and the onboard discrete measurements of the positioning and orientation. Flight trials are flown at an altitude of 8300 m over the Qinghai Lake China. It is shown that the image positioning accuracy (about 1~4 pixels) is better than previous methods (more than 7 pixels, [G. J. Jedlovec et al. NASA Technical Memorandum TM - 100352 (1989) and D. P. Roy et al. Int. J. Rem. Sens. 18(9), 1865 - 1887 (1997)]). It is also shown that the model has the potential to hold the image positioning errors within one pixel. The model can operate automatically, and does not need ground control points data. Since our algorithm get the image positioning results through an observation geometric perspective which is in computing the point at which the sensor viewing vector intersects the earth surface, our algorithm assumes the airborne data are from the plain area.