This study evaluated the use of Positive Behavior Support (PBS) based on Functional Behavioral Assessment (FBA) to reduce inattentive behaviors in children with moderate intellectual disabilities. Employing a multiple-baseline design, three 15-year-old male students from a special education school in Shandong participated in individualized interventions lasting 9–15 sessions. Strategies included classroom environment adjustments, behavior cues, differential reinforcement, and timely behavioral interruptions. Results showed substantial reductions in inattentive behaviors across all three cases, with Case J showing the greatest improvement (from 11.78 to 2.45 instances per session), followed by Cases W and G, with decreases of 6.44 and 4.97, respectively. All cases had a Tau effect size of 1.00, reflecting strong intervention effectiveness. Participants also displayed enhanced focus, self-regulation, and quality of life, as reported by teachers and parents. However, limitations such as short intervention durations and limited parent involvement were noted, pointing to the need for extended interventions and stronger home-school partnerships. Overall, PBS proved to be an effective and flexible approach to improving classroom behaviors and social adaptability in children with moderate intellectual disabilities.