Composition is an important approach to model larger and more complex workflow system. Task refinement is one kind of workflow composition approaches. Soundness is the important property as correctness criterion for workflow process model. In some times, the state space methods based on the reachability graph of Petri net is an available analysis technique. When the composite system becomes larger, the state space of the composite workflow net system becomes too large to analyze it. To avoid the state space explosion problem, invariant analysis can be a complement analysis technique. Based on the invariant analysis, we propose a sufficient condition for 1- soundness of task refinement of workflow nets.