Objective To investigate the present situation of 0~3 years old infants' bone mineral density (BMD) in Xi'an,and to explore the effect of age,gender and season on infants' BMD,in order to provide evidence for early prevention of bone strength deficiency. Method Quantitative ultrasound was used to check BMD in the left tibia middle point for 7 207 children aged 0~3 years old,and then ultrasonic bone mineral density values of different genders,ages and seasons were analyzed and compared. Results Bone mineral density speed of sound (SOS) value in girls was significantly lower than boys(P<0.001).There was significant difference in bone mineral density SOS and Z values for different month-old infants (P<0.001).The SOS values of 0~3 months were the lowest.And the Z values of 0~3 months and 3~6 months were no significantly difference (P>0.05).The overall incidence rate of bone strength deficiency was 74.29%,which in girls (81.10%) was significantly higher than that in boys (68.02%).The incidence rate of bone strength deficiency in 0~3 months and 3~6 months were higher than other ages.And it decreased with the age increased.The SOS value of boy in the winter was the lowest. Conclusions The infants and young children need to be supplemented calcium and vitamin D.The key period to prevent and treat of bone strength insufficiency is 0 to 6 months after birth,and girls are prone to bone strength insufficiency.