The science process is a process to solve a complex scientific problem which involves large numbers of resources and scientists. Organizing science processes in workflow forms and deploying such scientific workflows in Grid environments could provide the effective process management and resource sharing so as to achieve the automatic performance of science processes. However, most current workflow models rarely take account of specific characteristics of science processes and are not very suitable for scientific workflow modeling. Therefore, a new model special for science processes, named as the problem-based scientific workflow model, is proposed in this paper. It takes subproblems as basic modeling elements and puts forward three new relationships based on the problem logic analysis. Moreover, based on this model which conforms to characteristics of science processes, the mechanism about workflow performance in Grid environments is provided.