Cross-organizational workflow has some different characteristics compared with normal workflow, such as process-oriented, compositionality, abstraction, involving communication, and collaboration of several systems. Traditional workflow modeling methods cannot meet these new requirements because they don't have mechanisms to support abstraction, and there are no standards and concurrency operations to obtain bigger models by combining small ones in these modeling methods. Aimed at this problem, a cross-organizational workflow modeling method based on Pi-calculus is presented and the consistency and compositionality of the model are analyzed. Using concurrent operators Pi-calculus provided, a cross-organizational business process is modeled as a composition of a set of autonomy and concurrent intra-organizational sub-processes, and an intra-organizational sub-process is modeled as a composition of a local business process definition and control constraints between organizations as well. Based on weak bisimulation theory of Pi-calculus, external behavior equivalence of two cross-organizational sub-processes is defined, which can help building abstract of an internal private business process. Compared with the traditional methods, this new cross-organizational workflow modeling method builds a loosely coupled relationship among sub-processes, which makes this model adapt to dynamic cross-organizational environment. And this model is based on strict formal method, which facilitates the analysis and verification of business process models.