Based on the monthly precipitation data from 57 weather stations in the Loess Plateau from 1961 to 2015, the standardized precipitation index (SPI) was calculated as the index of drought and flood to study the circulation characteristics in abnormal drought and flood years in April in the Loess Plateau and their earlier forcing signals. The results are as follows: (1) The ridge from Ural Mountains to Xinjiang in China was stronger, and East Asian trough was deeper on 500 hPa, and the south wind was weaker on 700 hPa over the Loess Plateau and its east, and the temperature was  lower on 700 hPa from east China to Japan, and the westerly jet from east China to the ocean on the east of Japan moved southerly  and  became stronger. For these reasons, it became drought in April in the Loess Plateau, vice versa. (2) The frequency of the blocking in April in the range of 180°E-150°W in mid-high latitude of the Northern Hemisphere was more in the abnormal flood years than in the abnormal drought years. (3) The south Indian ocean dipole (SIOD) or the southwest Indian ocean SST could be used as a previous forcing signal for drought and flood in April. There was a significant negative correlation between SPI in April and SIOD of December in the previous years. (4) The responses of the circulation field to the positive and negative phase anomalies of SIOD were mainly reflected in the differences of the Ural high pressure ridge, the East Asian trough on 500 hPa and the circulations over the area from Lake Baikal to northeast China and Ural Mountains on 700 hPa.