A low-cost and ease-fabricated mode interference sensor for zinc ion detection is proposed in our work. The sensor is constructed by direct fusion splicing of single-mode fiber (SMF) and no-core fiber (NCF). Experiment and simulation results of the sensor exhibits consistent good response characteristics in the refractive index (RI) range of 1.3324-1.3791. The zinc ion sensitive film was functionalized on the sensor surface by electrostatic self-assembly of chitosan (CS) and polyacrylic acid (PAA), and the experimental results show the optimal detection performance with 12 layers of CS/PAA coated. RI changes in the CS/PAA polymeric film induced by adsorbing zinc ions, which affects the internal energy distribution inside NCF and eventually causes the wavelength shifts. Finally, the high sensitivity of 0.78 nm/ppb and lowest detection limit of 0.01 ppb for zinc ions detection were obtained. Furthermore, the sensing properties of cobalt, sodium and potassium ions were also investigated to evaluate the detection capability for different species metal ions. As a result, the proposed sensor shows well performance on the stability and sensitivity, it has good development prospect in the future water quality environmental monitoring.