In order to better analyze the possibility that the floating Sargassum horneri in East China Sea originated from benthic S. horneri in Zhoushan sea area, and provide an ecological reference for effective tracking and monitoring of floating Sargassum, morphological characteristics of vesicle of S. horneri and its relationship to environmental factors were investigated. The results showed that algae began to have vesicle in September, and have a highest growth rate from March to May. The number of vesicles in the main side branches increased rapidly from March to April, and reached a maximum in April. The number, average length, average wet weight and average volume of vesicles in each month outnumbered in top main side branch than those in middle and bottom main side branches, but the average diameter was relatively uniform. The net effective accumulated temperature needed by S. horneri to complete the life history was 4 434.75 ℃·d. Vesicle germinating net effective accumulated temperature in September was 1 853.47 ℃·d, and development completing net effective accumulated temperature in April was 3 611.25 ℃·d. The sea surface temperature change in the study area is mainly affected by the current changes caused by monsoon activity. The warming condition after a low temperature period of sea water may be the factor that induces the rapid growth of S. horneri, and higher sea temperature may limit algae growth.