Abstract: The authors have established a formula of drought and flood in agromet-eorolgy from the viewpoint of water-balance in farmland, and analysed the crops require-offering relationship and crops requirement of water (N), at the same period, that is. K={W+R(1-σ-S)+Q}/NMany parameters of effect have been considered, such as precipitation (R), moistures of soil in the depth of a metre (W), surface runoff (a), evaporation of under-ground water and moisture in the deep soil (Q), permeation (S) at the supply of water as the parameters of effect.As the parameters of effect are different in distinctive region and reasons, the parameter K has different formula of simplifying.The authors have established the table of drought-water-logging by experience (Table 5,) here K is divided into five grades i. e. water-logging, sub-waterlogging, normal, subdrought and drought.Finally, both K anb the real case are compared, it shows that they are approximate.