Solution:
The MI Band itself can store data separately, so there's no need to carry your phone when you're out running or sleeping. To synchronize the data, please turn on the phone's bluetooth and move it close to the Band to connect.
The body of the MI Band can retain the sports data information for about 7 days at most. Data that have been synchronized to the MI Fit App will be automatically uploaded to the cloud for storage. If a replace new phone, as long as the previous MI account is used on the new phone and the band is bound, the previous data can be synchronized from the cloud to the new phone.