Since the debut of Bitcoin, a blockchain platform, blockchain-based cryptocurrencies have received wide attention and accumulated a wealth of transaction data. These transaction records include rich information and complete traces of financial activities, and therefore provide us an unprecedented opportunity for knowledge discovery. Networks are a universal language for describing interacting real systems, and much work on cryptocurrency transactions is conducted from a network perspective. This survey summarizes the existing work on analyzing and understanding blockchain transactions, aiming to provide a systematic guideline in this area. We first introduce the background of blockchain transaction, and then review existing research in three different aspects, i.e., transaction network modeling, transaction network analysis, and network-based detection technology, the purpose being to provide a systematic guideline for researchers in this area.