Recently, eXtended Reality (XR) has gained significant attention for providing immersive user experience in various applications with the help of 5G new radio (NR) network. To ensure the quality of experience (QoE) in XR services, wireless transmission plays an important role. In this paper, we propose a novel performance metric that can reflect the impact of network transmission on XR services, naming it as XR Quality Index (XQI). Specifically, fine-grained and coarse-grained XQI models are provided with detailed calculation procedures. The aim of XQI is to produce a final score that can reflect the impact of network transmission on QoE, and then could be used for network optimization. Simulation results verify the effects of XQI in evaluating the quality of XR videos.