Objective　To investigate the correlation between the ratio of thorax diameter and spontaneous pneumothorax (PSP) in males.　Methods　Forty-four male patients with PSP were admitted in the department of thoracic surgery of the central hospital of Wuhan from July 2017 to June 2018. The control group was 30 healthy men of the same age. CT data of chest were collected, and the lung and thorax were reconstructed with Mimics Medical 20.0 software to calculate the lung height H. The maximum transverse diameter T (maximum distance between the left and right sides of the inner wall of the thorax) and the anterior and posterior diameter D (vertical distance from sternum to spine in the plane) were measured respectively at five planes (plane of first costal cartilage, sternal angle, carina, end of sternum and end of xiphoid). Flat index (FI=T/D), slim index (SI=H/D), height transverse ratio (HTR=H/T) were compared between the two groups. Correlation analysis of pneumothorax occurrence was performed on the values with significant differences, and binary logistic regression analysis was performed on the greatest related factor.　Results　Significant differences were showed in the diameter ratios between the two groups. According to age stratification, SI5 (the plane of the xiphoid end) was the most important risk factors for PSP (R=0.682, R2=0.465, P<0.01). The predictive accuracy of SI5 was 82.40%, OR was 276.49, 95% confidence interval was 21.20~3606.67(P<0.01).　Conclusion　The SI at the end of xiphoid is the most relevant factor for PSP. Compared with the degree of thoracic flatten, the slim degree is more correlated with PSP. Smaller anteroposterior diameter of the xphoid region is probably one of the pathogenic factors.