We present the study of stellar populations in the central 5.5′ ( \sim 1.2 kpc ) of the M31 bulge by using the optical color magnitude diagram derived from HST ACS WFC/HRC observations . In order to enhance image quality and then obtain deeper photometry , we construct Nyquist-sampled images and use a deconvolution method to detect sources and measure their photometry . We demonstrate that our method performs better than DOLPHOT in the extremely crowded region . The resolved stars in the M31 bulge have been divided into nine annuli and the color magnitude diagram fitting is performed for each of them . We confirm that the majority of stars ( > 70 % ) in the M31 bulge are indeed very old ( > 5 Gyr ) and metal-rich ( [ Fe/H ] \sim 0.3 ) . At later times , the star formation rate decreased and then experienced a significant rise around 1 Gyr ago , which pervaded the entire M31 bulge . After that , stars formed at less than 500 Myr ago in the central 130″ . Through simulation , we find that these intermediate-age stars can not be the artifacts introduced by the blending effect . Our results suggest that although the majority of the M31 bulge are very old , the secular evolutionary process still continuously builds up the M31 bulge slowly . We compare our star formation history with an older analysis derived from the spectral energy distribution fitting , which suggests that the latter one is still a reasonable tool for the study of stellar populations in remote galaxies . Keywords : galaxies : bulges < Galaxies , galaxies : evolution < Galaxies , galaxies : individual ( M31 ) < Galaxies