The embodiment of the present invention discloses a convergent online billing method and system, wherein the method comprises: a user switches from a first network to a second network, a service identification gateway sends an online billing system a first available limit for the user in the first network and information about a new network access type after the user switches to the second network; the online billing system conducts billing for the user according to a first billing mode and the first available limit and acquires a second billing mode corresponding to the new network access type; the online billing system sends information about the second billing mode and second limit information to the service identification gateway; in the second billing mode, the service identification gateway conducts available limit monitoring based on the second limit information and reports a second available limit for the user in the second network to the online billing system; and the online billing system conducts billing for the user according to the second billing mode and the second available limit. The embodiment of the present invention can satisfy actual service demands and avoids user billing errors.