Privacy preservation in wireless sensor networks has attracted more and more attentions recently. Computing top-k query result while preserving data privacy is a challenge in wireless sensor networks. This paper proposes a prefix membership verification (PMV) based privacy-preserving top-k query processing in two-tiered wireless sensor networks, which is denoted as PPTQ. In PPTQ, each sensor node in the query aera firstly converts its collected data into encoded ones by using prefix encoding and hashed message authentication encoding mechanism, and encrypts all the collected data items, then submits the encoded data and the ciphertext to the corresponding storage node. According to the numerical comparison theory of PMV, the minimum candidate ciphertext sets containing the query result will be generated by the covered storage nodes, without knowing the actual values of the collected data items. Then, the storage nodes send the candidate ciphertext to sink, and the final top-k result will be computed by sink after decryption. To reduce the power consumption of sensor nodes, an energy optimization method based on a Hash function is proposed. The theoretical analysis and experimental results show that the PPTQ can ensure the privacy of the collected data and the query result, and it has better performance than the current work in query processing.