With the aim of achieving the simultaneous monitoring of changes in seawater temperature and salinity, we propose a surface plasmon resonance (SPR) sensor based on photonic crystal fibers (PCFs). The sensor probe is realized by fusing a segment of PCF between two segments of multimode fiber and plating a gold film on the surface of the PCF to stimulate the SPR phenomenon. This scheme combines the fiber optic SPR sensing probe with the temperature-sensitive material polydimethylsiloxane to form a dual SPR effect, achieving the simultaneous detection of the temperature and salinity of seawater. The experimental results show that the maximum temperature sensitivity of the sensor is－2.021 nm/℃, and the maximum salinity sensitivity is 0.418 nm/‰. This sensor has a small volume, simple production, and excellent performance. It is suitable for multi-parameter and distributed measurements of seawater and has good application prospects in liquid substance measurement.