Although the system throughput of a wireless communication system considering multiple users' geographical distribution on a 2-D plane and various traffic characteristics is an important performance metric, its mathematical analysis is very difficult due to the nonindependent identically distributed distribution of the received signal-to-noise ratio (SNR) of each mobile terminal and the complex dynamics of the traffic patterns. Therefore, complicated system-level simulations have been the only approach to evaluate the system throughput of downlink packet-access systems. In this paper, we propose an efficient analytical approach to estimate the system throughput of downlink packet-access systems by using point mass approximation, which is widely used in physics and mechanics. The proposed analysis framework enables the mathematical analysis of the system throughput performance by considering various positions of mobile terminals and traffic patterns without the complicated and heavy system-level simulations. Although the proposed approach has a tradeoff relation between computation complexity and accuracy, we show that it is possible to obtain an effective accuracy with moderate complexity using the proposed analytical approach.