A growing degree-day (GDD) calculation method was recommended by comparing several popular used GDD calculation equations. The GDD between different development stages, from emergence to each leaf appearance and during the lifetime of each leaf, were calculated for summer maize with the field treatments differed in cultivars, plant density, sowing dates, water and fertilizer supplying levels. Factors influencing the stability of GDD were discussed, and simulation equations to predict the leaf development were fitted based on the field observed data.