A data update method, a client, a server and a system for data update are provided. Identifiers and update priorities of data strings to be updated in an application data packet are acquired. The data strings to be updated are classified, according to a predetermined threshold, as data strings to be updated having priority higher than the predetermined threshold and data strings to be updated having priority lower than or equal to the predetermined threshold. Before the application data packet is started, update data of the data strings to be updated having priority higher than the predetermined threshold are acquired and updated. After the application data packet are started, the update data of the data strings to be updated having priority lower than or equal to the predetermined threshold are acquired and updated.