Aiming at the problem that the traditional topic model ignore short text and dynamic evolution of microblog, a Biterm Topic over Time (BToT) model based on microblog text was proposed, and the subject evolution analysis was carried out by the proposed model. A continuous time variable was introduced to describe the dynamic evolution of the topic in the time dimension during the process of text generation in the BToT model, and the "Biterm" structure of the topic sharing in the document was formed to extend short text feature. The Gibbs sampling method was used to estimate the parameters of BToT, and the topic evaluation was analyzed by topic-time distributed parameters. The experimental results on real microblog datasets show that BToT can characterize the latent topic evolution and has lower perplexity than Latent Dirichlet Allocation (LDA), Biterm Topic Model (BTM) and Topic over Time (ToT).