A biosorbent was prepared from the cellulose fibers found in rice straw through cationic modification. The effects of the dosage, pH, contact time, and initial concentration of lemon yellow dye were explored. The static adsorption results showed that cationic modification drastically improved the adsorption capacity of straw cellulose fiber. The maximum equilibrium adsorption capacity value was 137.6 mg/g and the highest removal reached 99%. The pseudo-second-order kinetic model was a good fit for the adsorption process, together with the Langmuir isotherm model. The adsorption reaction was spontaneous, and the adsorption process was an exothermic reaction, which was shown by the thermodynamic model. As the adsorption time became longer, the effluent concentration became larger until reaching equilibrium. The time was 420 min. After desorption using a dilute NaOH solution, the maximum adsorption capacity was still 36.1 mg/g and the maximum removal still reached 36.2%. The parameters calculated from the Yoon-Nelson model have a good fit with the experimental data. In short, cationic straw cellulose fiber is an effective and easy to prepare biosorbent. This work offers a new method for dye wastewater purification and solves the effective utilization of rice straw resources.