The historical climate data, unit yield data as well as observed crop developmental phases information were used, then the variation trend of climatic elements for different growth period and relative climatic yield were researched. By method of different basic periods dividing, the influence rate of climate change on unit yield of summer maize in Henan province was computed. The results indicated that, for representative stations, the average air temperature in the vegetative growth period and the whole growth stage varied as cubic polynomial, the sunshine hours declined significantly for both the vegetative and reproductive growth phase, while the precipitation fluctuated severely. Relative climatic yield of summer maize varied in stability between the years of 1961 and 1981, and there was a notable negative correlation between relative climatic yield and average air temperature for the vegetative growth phase. After 1988, interannual variability of relative climatic yield increased obviously, and showed a notable negative correlation with precipitation of the reproductive growth phase. Compared with the yields of 1961-1981, since the 1990s, the influence rate of climate change on unit yield of summer maize was between -2.1% and -9.3%, and compared with the yields in 1991-2000, since the 21 century, the climatic conditions had advantage impact on yield formation, and the influence rate was 4.7%.