PurposeThe purpose is to forward systems theory one more step towards social theory and integrate problem-solving and theory-building, and search for the integration and unity of science by revealing the nature and role of critical systems thinking (CST).Design/methodology/approachThis article describes relations between systems theory and social theory in three parts. First, it examines the links of systems methodologies with three social science approaches as well as the role of CST. Second, the focus of theory and the form of explanation are discussed from critical social science (CSS) perspective. Third, the direction of theorizing of a CST-based systems theory is investigated.FindingsFirst, CST is a hidden assumption of system dynamics (SD)/systems thinking (ST). Second, systems theory is positioned in CSS. Third, CST integrates traditional and soft systems methodologies (SSM), and connects systems science and social science. Fourth, this article reveals hidden links between systems approaches and three corresponding social science approaches. Fifth, the theoretical focus of a CST-based systems theory could be formal/structure theory and/or substantive/content theory. Sixth, the form of explanation could be structural/mechanismic explanation combining causal and interpretive explanations. Seventh, a CST-based systems theory may adopt abduction, which complements a defect in deduction and induction in a difficulty of nonlinearity.Originality/valueIt illustrates a graph of the competing approaches in systems science corresponding to paradigms in social science.