Objective To explore the levels of smartphone addiction among medical students and to identify the influencing factors. Methods A total of 493 medical students from a university in Shanxi Province were investigated with the Mobile Phone Addiction Index （MPAI）, Cognitive Appraisal Orientation Test （CAOT）, Self-rating Depression Scale （SDS） and Self-rating Anxiety Scale （SAS）. Results The rate of smartphone addiction in medical students was 66.32%, and the score of mobile phone addiction was （43.05±10.10）.Smartphone addiction was positively correlated with pessimism, anxiety and depression, and negatively correlated with optimism （P＜0.05,P＜0.01）. Logistic regression analysis showed that no examinations in the last week (OR=1.782), and depression (OR=1.096) contributed to smartphone addiction (P＜0.05 for both). Conclusion The rate of smartphone addiction in medical students is high. The mental states of medical students need regular assessment. Measures should be taken to help medical students to adjust and release their emotions, and agreeably express their feelings, which could improve their self-discipline, and make them use smart phones reasonably and wisely.