A method of calculating broadband access server DHCP user's on-line time, includes these steps: (a) setting the inner time, outer time and traffic threshold for said access server calculating user's data traffic, and the inner time is shorter than the outer time; (b) user accesses the said access server, after authenticated successfully, the access server notifies starting to charge the user's online time; (c) according to each user, said access server establishes a ring list to record data traffic; (d) said access server sets the number of ring list elements as multiple of said outer time to said inner time; (e) said access server inspects the said user's data traffic according to inner time, fills it in content indicated by top element index of ring list, until current data traffic inspected and value recorded by top index are not more than said traffic threshold. The advantage of the invention is that timing is precise and error is small.