Existing routing protocols for multi-sink wireless sensor networks (WSNs) attempt to optimize energy efficiency and routing survivability from the perspective of the network itself, without considering the impact of external environment, making them unable to respond to environmental changes in a timely manner. Therefore, the routing survivability of these routing protocols in harsh environments is questionable. To solve this problem, we design a sustainable multipath routing protocol SMRP, in which the routing decisions are made according to a mixed potential field in terms of depth, residual energy and environment. The basic idea of SMRP is to instruct messages to select paths with a tradeoff among delivery latency, energy balance and routing survivability. As the environmental field is constructed and updated using the sensing capability of WSN itself, the constructed multipath can be secured by avoiding passing through the dangerous areas. We explore the impact of the number of sink nodes and the number of paths on routing performance, and compare SMRP with commonly used routing protocols (i.e., EDGR and IPF). Through extensive experiments, SMRP has proved to have better performance in terms of packet delivery ratio and network lifetime under harsh environments.