IEEE 802.15.4 protocol is proposed to meet the low latency and energy consumption needs in low-rate wireless applications, however, few analytical models are tractable enough for comprehensive evaluation of the protocol. To evaluate the IEEE 802.15.4 slotted CSMA/CA channel access mechanism in this paper, we propose a practical and accurate discrete Markov chain model, which can dynamically represent different network loads. By computing the steady-state distribution probability of the Markov chain, we obtain an evaluation formula for throughput, energy consumption, and access latency. Then we further analyze the parameters that influence performance including packet arrival rate, initial backoff exponent and maximum backoff number. Finally, NS2 simulator has been used to evaluate the performance of the 802.15.4 CSMA/CA mechanism under different scenarios and to validate the accuracy of the proposed model.