ADHD is a common behavioral disorder characterized by the symptoms of inattention , hyperactivity and behavior impulse. While ADHD is a group of diseases which often occur in combination with one or more mental or non-mental disorders. Previous studies on comorbidities of ADHD mostly confine to symptoms of one or several comorbities, so we don’t have a system understanding of this issue. Comorbidities of ADHD can be divided into three parts according to the temporal order of occurrence. That’s pre-comorbidity ,simultaneous comorbidity and post-comorbidity. Pre-comorbidities present before the appearance of core symptoms of ADHD. For example, disturbance of sleep and autism spectrum disorders (ASD) may precede first clinically relevant ADHD symptoms. Simultaneous comorbidities coincide with the time when ADHD symptoms reach a clinically significant level. Researchers find that some people with ADHD are diagnosed with non-organic enuresis and encopresis when ADHD symptoms reach a clinically significant level. What’s more, develop-mental dyslexia which is characterized by difficulties with reading comprehension, reading decoding, reading fluency and spelling, may manifest as simultaneous comorbidity with ADHD. Post-comorbidity is an important part of these comorbidities. These disorders in people with ADHD often occur several years after the appearance of core symptoms. For example, some people with ADHD have related symptoms of depression later on in the course of ADHD. As depression, researchers find that people with ADHD may also show symptom of anxiety disorders, OCD, CD, ODD and substance use disorder later on in the course of ADHD. On the basis of this classification, researchers will have a relatively systematic understanding of this issue. They will also have a more specific aim in the future studies. In this way , future studies will provide researchers with more effective material for the diagnosis and treatment of ADHD. To understand the development of comorbidity and do better in the diagnosis and treatment of ADHD, longitudinal study in patients with ADHD should be strengthened in the future.