Based on the principle of silicon surface film passivation, the effect of different thickness surface passivation film on the electronic sensitivity of back-thinned CMOS (BCMOS) sensor was studied. Firstly, the electron bombardment test was carried out after the back thinning processing on CMOS sensor. The electron bombardment test shows that the gray value of the electronic image presents a linear relationship with the change of bombarding electron energy. Then, the aluminum oxide films with different thickness were deposited on the surface of back-thinned CMOS, and the electron bombardment test was carried out. It was found that the collection efficiency of secondary electron was increased by 14.9% when the thickness of aluminum oxide film was 20 nm, meaning that the electron sensitivity could be improved by surface film passivation. Furthermore, the dark current of the back-thinned CMOS sensor reduced from 1510 e^-/s·pix^?1 to 678 e^-/s·pix^?1 with the increase of film thickness. The above results show that aluminum oxide film has a good passivation effect on back-thinned CMOS sensor, which could improve the secondary electron collection efficiency and reduce the dark current of the back-thinned CMOS sensor, and provide a technical support for the development of high sensitivity EBCMOS devices in the future.