A polymer optical fiber SPR sensor for detecting Cu²⁺ ion concentration in water is proposed. The sensor employs a simple side-polish structure and realizes the detection of Cu²⁺ ion concentration by employing the chitosan (CS)/polyacrylic acid (PAA) bilayer film on the gold film of the optical fiber surface. The structure of the fiber probe is optimized, and the sensing performances for the Cu²⁺ ion detection are analyzed experimentally. The experimental results demonstrate that the sensor exhibits a high sensitivity of 465.539 nm/ppm for the Cu²⁺ ion detection in the concentration range of 0–0.04 ppm. And it has a fast response speed and good selectivity for Cu²⁺ ions. The sensor has the advantages of simple structure and low cost, and has potential applications in the field of heavy metal detection.