The traditional inertia monitoring method designed is based on a continuous physical model, converges slowly and it is difficult to accurately track the virtual inertia with time-varying characteristics. Considering the characteristics of discrete data output from modern power system measurement devices, an online monitoring method of inertia at power system convergence points based on a discrete information model is proposed. First, based on a mathematical model containing inertia constants in the continuous time system, an equivalent motor discrete transfer function in the Z domain is constructed. Second, based on the mathematical structure of the transfer function in the Z domain, a regression method is used to construct another equivalent motor discrete transfer function. Based on this, a method of calculating the equivalent inertia constants of the convergence point is proposed. Finally, the proposed method is validated using a simulation with the improved IEEE9- node system and it verifies the timeliness and accuracy of the proposed method. The results show that this online monitoring method of inertia based on the discrete information model can accurately measure the rotational and virtual inertia of the power convergence point under the full operational condition.