Abstract: Agricultural drought and flood disasters occur most frequently in the Huaihe River Basin of China, accounting for 80% of the total loss of grain production. Taking the Huaihe River Basin as the study area, this study aims to dynamically monitor the spatial and temporal pattern of farmland drought and flood for the main food crops, thereby improving the rational allocation for the regional water resources at the timeliness and precision level. The daily meteorological observation and soil moisture data were collected at the 173 meteorological stations from 1971 to 2020. The budget of farmland water was optimized to calculate the Standardized Antecedent Precipitation Evapotranspiration Index (SAPEI), according to the relevant characteristics of regional soil and main planting crops. The timeliness and refinement level of SAPEI was satisfied with the commercial requirements for the surface dry and wet changes in most areas, considering the farmland water balance and rational assessment of the drought and flood at multiple time/space scales. The applicability of SAPEI was finally evaluated on the temporal and spatial characteristics of farmland drought and flood over the past 50 years. The results showed that the probability distribution fitting of three parameter log-logistic was applicable to the SAPEI in the study area. The annual SAPEI showed an upward trend as a whole, indicating that the drought slowed down. There was a downward trend with an abrupt change in 1992 from the normal to the drought during the growth period of winter wheat. There were also some outstanding positive and negative fluctuations during the growth period of single cropping rice. For example, the SAPEI had achieved a minimum of -0.921 in 1978, where the continuous drought appeared from spring to autumn in the planting area of single cropping rice in the southern study area. More importantly, the precipitation was generally only half of water demand, where the yield of rice decreased significantly, indicating a very serious drought. There was an upward trend during the growth period of summer maize, where the index was continuously greater than 0 from 2001 to 2010, indicating an outstanding wet period. In addition, there was a moderate trend of drought at most stations in the spatial distribution. A case study showed that the SAPEI truly represented the daily changes of surface farmland drought and flood, 93% of the samples with the Kappa coefficient over 0.6, indicating better consistency. Consequently, the SAPEI can be applied to daily dynamically monitor the service of farmland drought and flood, thereby timely preventing and reducing the impacts of the farmland drought and flood on the crops in the Huaihe River Basin. This finding can also provide a strong reference for the decision-making on disaster prevention and reduction measures.