Abstract: The yield formation of some early-mature cotton varieties, as CCCR45 and 33B, and of some middle-mature cotton ones, as CCCR41 and sGK321, were analyzed in rainy autumn days during 2003 to 2004 year. The experiment results showed that the mildew and rot boll grew in number, plant decrepitude shifted to an earlier time and became heavier in the cloudy weather and rainy day of later season. The useful cotton boll rate at upper and outer parts on the plant was less than 10%, rot cotton boll rate of early-mature cotton was raised by 25.4%, which was 4 times higher than that of middle-mature one. In this case, the dominance of the precociousness of early-mature cotton had lost completely. Before 10 October, the harvest yield number of middle-mature cotton, which had clear superiority of boll formation on July, had increased 31.1% than that of early-mature one. Having the disposition of branches and leaves had to make a display of open and clear characteristic, the middle-mature cotton was identical with the optimum boll formation period of middle of Shaanxi province, which had good adaptability to the ecological environment of autumn-rainy days frequently being occurrence in this cotton region.