This paper proposes a design for a dielectric-grating-coupled surface plasmon resonance (SPR) sensor that can be fabricated using a low-cost nanoimprint process and exhibits a high phase detection sensitivity when light is incident on the backside of the sensor and does not pass through the analyte on the front-side of the sensor. A low-refractive-index material (mesoporous silica) is utilized to implement a reverse symmetric waveguide structure that can enhance electric-field strength on the sensor surface and improve detection sensitivity. A sol-gel method is used to fill the groove of the grating structure with a high-refractive-index material (titanium dioxide), and surface smoothness is improved via a flat silicon impression mold. The experimental results indicate that although the sensor device exhibits defects and non-smooth surface relief, phase detection sensitivity can still be achieved as high as 2 × 10-5 RIU by using an electro-optic heterodyne interferometer.