The invention discloses a on-line charging method comprising: determining insufficiency of network resource remaining quota of prepaid user; transmitting message carrying on-line charging indication information to prepaid client equipment; the on-line charging indication information comprising a redirecting rule for redirecting a service flow to charging service equipment; after the prepaid client equipment receiving the message carrying on-line charging indication information, then redirecting the service flow to charging service equipment to complement on-line charging. Accordingly, the invention also discloses on-line charging system and equipment. The invention enables the prepaid client equipment to use a service flow to complete charging for prepaid user without interrupting other service of the prepaid user, thus improving user experience.