This paper analyses potential maize productivity in Hailun, Heilongjiang by combining PS123 crop growth model with ordinary methods. Data from 1999 to 2001 were adopted to calculate the potential maize productivity and get the average as the final result. The estimates of photosynthesis potential productivity (PPP), the temperature potential productivity (TPP), climate potential productivity (CPP) and land potential productivity (LPP) of maize are 54008, 11998, 9531, 8006 kg/hm², respectively. Compared with the reality on maize productivity in Hailun, the use efficiencies of PPP, TPP, CPP and LPP are 10.7％, 48.3％, 60.9％, 72.4％, respectively, which means that there is still great potential in Hailun.