Network calculus is an elegant theory for performance analysis in computer network. in this paper, we try to improve network calculus so that it can be used to analyze the performance of network coding. network coding encodess multiple flows together at forwading nodes to improve the network throughput, raising a problem on identifying isolated flows at the nodes. we propose an analysis model to identify different flows at a coding node so that we can apply the flow-based theorems of network calculus. based on the model, we figure out the theoretical difficulties of network calculus. to tackle the difficulties, we improve network calculus by redefining its fundamental concepts. with the new concepts, we derive some theorems for the performance analysis in network coding. additionally, the practical issues on applying the theorems in real network analysis are discussed. a case study shows that our model can property analyze the performance of network coding.