The invention discloses a method and a system for estimating the capacity of a server. The method disclosed by the invention comprises the following steps of: counting the business-processing capacity finished by the server within unit time and the time length at the peak period each day; calculating the percent of the processing capacity of the server at the peak period accounting for the all-day processing capacity and the complexity parameter; adjusting the redundancy factor of the server and the utilization ratio of a central processor; and estimating the capacity of the server according to the business-processing capacity finished by the server within the unit time, the time length at the peak period each day, the percent of the processing capacity at the peak period accounting for the all-day processing capacity, the complexity parameter, the redundancy factor and the utilization ratio of the central processor. The invention also discloses the system for estimating the capacity of the server. Through the introduction of the complexity parameter in the invention, the differences of different application systems are eliminated, and a universal estimating method for the capacity of the server of each application system is realized, so that the estimating efficiency is improved; and estimation is carried out by utilizing the actual data provided by the current online application system, so that errors between the estimated capacity and the actual capacity are reduced.