The poor performance of the original BERT for sentence semantic similarity has been widely discussed in previous works. We find that unsatisfactory performance is mainly due to the static token embeddings biases and the ineffective BERT layers, rather than the high cosine similarity of the sentence embeddings. To this end, we propose a prompt based sentence embeddings method which can reduce token embeddings biases and make the original BERT layers more effective. By reformulating the sentence embeddings task as the fillin-the-blanks problem, our method significantly improves the performance of original BERT. We discuss two prompt representing methods and three prompt searching methods for prompt based sentence embeddings. Moreover, we propose a novel unsupervised training objective by the technology of template denoising, which substantially shortens the performance gap between the supervised and unsupervised setting. For experiments, we evaluate our method on both non fine-tuned and fine-tuned settings. Even a non fine-tuned method can outperform the fine-tuned methods like unsupervised ConSERT on STS tasks. Our fine-tuned method outperforms the state-of-the-art method SimCSE in both unsupervised and supervised settings. Compared to SimCSE, we achieve 2.29 and 2.58 points improvements on BERT and RoBERTa respectively under the unsupervised setting.