To theoretically and quantitively describe the phenomena that bone mineral density (BMD) was changed with, the three-exponential model was proposed. Our results showed that BMD in radius and ulna in a sample of 36,879 healthy people ranging in age from 2 to 100 years fit the three-exponential equation well. Bone mass loss in Chinese women was faster than that in Chinese men. The method to accurately calculate the peak BMD and the corresponding age was given. The peak BMD and age for Chinese men and women in radius were 0.7469 g/cm2 and 35 years old, and 0.7051 g/cm2 and 33 years old respectively. The peak BMD and age in ulna were 0.7515 g/cm2 and 35 years old for Chinese men, and 0.6904 g/cm2 and 31 years old for Chinese women. Peak BMD in Chinese women was lower than in men. Peak BMD occurred slightly earlier in Chinese women. We concluded that our model could successfully characterize the regularity of bone mass accumulation and loss during the whole lifetime. The quantitive relationship was very useful in the diagnosis of osteoporosis.