Objective To investigate the relationship between hepatitis B surface antigen (HBsAg) levels and liver pathology at different phases of natural history in chronic hepatitis B (CHB) patients, and to establish a non-invasive liver fibrosis diagnostic model based on HBsAg quantification.Methods A total of 145 CHB patients were enrolled and underwent liver biopsy from January 2013 to January 2015, among which 73 patients were hepatitis B e antigen (HBeAg) positive.HBsAg levels and HBV DNA levels were compared between patients at different phases of natural history and between patients with different HBeAg statuses.Logistic analysis was used to analyze the risk factors associated with fibrosis in HBeAg-positive patients, and to evaluate the predictive value of non-invasive liver fibrosis diagnostic model based on HBsAg quantification.Analysis of variance was used for statistical analysis, and t test analysis was used for the comparison between two independent samples.Results The serum HBsAg levels at the immunologic tolerance phase, immunologic clearance phase, low copy phase and reactivation phase of CHB patients were (4.29±0.69), (3.56±0.61), (3.22±0.64), and (3.54±0.50) lg IU/mL, respectively (F=16.72, P<0.01), and the HBV DNA levels were (8.48±0.58), (6.69±1.44), (3.80±0.59), and (6.21±1.06) lg IU/mL, respectively (F=76.73, P<0.01).In HBeAg-positive CHB patients with liver inflammation stage (G)≤G1, G2, G3 and G4, the serum HBsAg levels were (4.44±0.65), (4.00±0.72), (3.74±0.62), and (3.28±0.50) lg IU/mL, respectively (F=9.198, P<0.01).In HBeAg-positive CHB patients with liver fibrosis stage (S)≤S1, S2, S3, and S4, the serum HBsAg levels were (4.55±0.54), (4.04±0.89), (3.59±0.63), and (3.34±0.50) lg IU/mL, respectively (F=10.66, P<0.01).Logistic analysis showed that age (OR=1.091, 95%CI: 1.013-1.175) and HBsAg level (OR=0.190, 95%CI: 0.066-0.542) were independent factors for predicting fibrosis stage.The area under receiver operating characteristic curve of the non-invasive fibrosis model based on age and HBsAg level was 0.849, which was higher than aspartate aminotransferase to platelet ratio index (0.749) and fibrosis index based on the 4 factors (0.763).Conclusions The serum HBsAg levels are significantly different among the different phases of natural history in CHB patients.The serum HBsAg levels decline with the progression of liver fibrosis in HBeAg-positive CHB patients.The non-invasive diagnostic model that based on HBsAg quantification could be used to evaluate the stage of liver fibrosis.