Adolescent idiopathic scoliosis is becoming a common spinal disorder among adolescents. The traditional methods of scoliosis screening are labor-intensive and can result in unnecessary referrals and radiological exposure for adolescents due to their low positive predictive value. For early screening of scoliosis and abnormal posture, a mobile-based cost-free, accurate and radiation-free scoliosis screening system is proposed in this paper. We establish a database with labeled 2D unclothed back images and corresponding whole-spine standing posterior-anterior X-ray images, and innovatively propose a new network topology of the 2D back image to localize the back landmarks. With only an unclothed back image, this system can automatically classify normal, abnormal posture and scoliosis with an overall classification accuracy of 88.1%. This system has the potential to overcome the time and space limitations of conventional screening for scoliosis and abnormal posture.