Abstract: In order to evaluate the photometric precision of the 1m telescope of the Weihai observatory, we carried out a test in which the open cluster M39 was observed in October 7, 2009. As several stars are particularly crowded in M39, we used the DAOPHOT package in the IRAF to measure their luminosities and directions. This paper describes how to use the DAOPHOT package in detail. The results show that for the stars successfully matched to the UCAC3 catalog, the photometry of bright stars has an internal precision of 0.003mag, and the photometry of faint stars in dense regions has an internal precision of 0.0128mag.