To address the issue of one-to-many mapping from phoneme sequences to acoustic features in expressive speech synthesis, this paper proposes a method of discourse-level prosody modeling with a variational autoencoder (VAE) based on the non-autoregressive architecture of FastSpeech. In this method, phone-level prosody codes are extracted from prosody features by combining VAE with FastSpeech, and are predicted using discourse-level text features together with BERT embeddings. The continuous wavelet transform (CWT) in FastSpeech2 for F0 representation is not necessary anymore. Experimental results on a Chinese audiobook dataset show that our proposed method can effectively take advantage of discourse-level linguistic information and has outperformed FastSpeech2 on the naturalness and expressiveness of synthetic speech.