ing at the problem that the existing surface plasmon refractive index sensor has a small vertical detection depth and the detection range cannot cover the entire thickness of cells, we propose a real-time measurement method of the living cell refractive index for its advantages: large detection depth and high sensitivity. And this method is used to carry out the experimental research on drug susceptibility. Based on the polarization-selective absorption effect, we design and build a graphene-based refractive index sensing system under the condition of total internal reflection. The refractive index with various mass fraction of sodium chloride solution is measured. The results indicate that the sensitivity and resolution of the system is 9.5×10 ⁶ mV/RIU and 5.5×10 ^-7 RIU, respectively. The experimental study on the drug susceptibility of living cells is carried out by the system. The real-time changes of cell refractive index during the biological evolution of cisplatin and paclitaxel in Ramos cells and Jeko-1 cells are studied, and the consistency of refractive index changes with its pharmacological mechanism is verified.