Abstract: An index series of mean year-to-year categories for drought and flood in middle reaches of Changjiang river valley, reconstructed using historic data and raingauge records for the past 100 years, is used to study the variations. It is found that flooding mostly occurs in the early part of this century while drought dominates the latter half; there are quasi-periodic oscillations of 22 years, 5-6 years and 2-3 years; drought and flood alternate in 6 stages.