The temporal changes of harmful algal blooms (HABs) indicate the phenological characteristics of algal blooms to some extent, the study of the phenological variations in HABs can provide a scientific reference for lake management and ecological restoration. Previous studies with MODIS data cannot monitor the situation before 2000, largely limiting the understandings on the spatiotemporal variation in HABs in Lake Chaohu. Here, we employed heritage Landsat missions and MODIS data to extend the time series of HABs based on the floating algae index and a threshold segmentation technique, the spatiotemporal changes and driving factors of HABs in Lake Chaohu from 1987 to 2020 were analyzed on the basis of ensuring the consistency of HABs results between Landsat and MODIS. The results showed that: (1) Comparing to 1987-1999, the area of algal blooms increased significantly after 2000 and a peak of 608.4 km² was found in 2011. (2) The temporal variations in cyanobacterial blooms in Lake Chaohu were divided into three stages in the period of 1987-2020. From 1987 to 2004, the annual start time of HABs was earlier significantly, while the duration of HABs increased significantly. The start time of the outbreak delayed significantly, and the duration of HABs did not change significantly from 2005 to 2010. From 2011 to 2020, the start time, termination time, and duration of HABs showed fluctuating changes with an insignificant decrease. (3) Changes in the timing of HABs in Lake Chaohu were mainly related to climatic factors such as temperature, precipitation and sunshine duration and were influenced by nutrients as well. During 1987-2004, the earlier start of the outbreak was dominated by the mean annual temperature in the previous year. From 2005 to 2010, the delay in the start of the outbreak was due to the combined effect of temperature, sunshine and precipitation in the spring; the earlier termination time and shorter duration of outbreak were associated with annual precipitation. This study established a four decades of long-term series variations of algal blooms in Lake Chaohu which provided insights to the spatio-temporal characteristics and driving forces of cyanobacterial blooms and largely improved the scientific disposal and response of algal blooms under the context of climate changes.