Abstract: Biodiversity conservation effectiveness assessment is an important basis for evaluating the effectiveness of conservation measures, optimizing resource allocation, and supporting adaptive management. However, existing assessments still face a methodological dilemma in baseline specification. Historical baselines, natural conditions, reference states, target states, and counterfactual scenarios are often used interchangeably, although they serve different evaluative functions. This conceptual ambiguity makes it difficult to distinguish whether biodiversity has improved, whether an ecosystem has been restored, whether policy targets have been achieved, or whether interventions have been effective. Climate change, land-use transformation, and social-ecological reorganization further weaken the applicability of a single historical baseline. Building on existing reference-state research, this paper proposes the concept of the contemporary reference state. The contemporary reference state refers to a comparable, interpretable, and dynamically adjustable reference condition identified under current and foreseeable climatic, land-use, ecological-process, and social-ecological constraints. It is derived from historical records, analogous reference systems, long-term monitoring data, model simulations, and expert judgment. The concept is intended to address limitations of existing reference-condition frameworks in the presupposition of reference attainability, applicability across contexts, and coverage of assessment functions, so that constraints are closer to reality, assessment functions are more systematic, and application contexts are broadened. Based on this concept, the paper develops an assessment framework that makes baseline specification an explicit and independent step. The framework includes five components: defining assessment objects and conservation goals, identifying the contemporary reference state, constructing indicators and integrating multi-source evidence, judging conservation effectiveness, and adaptive feedback. It also sets out operational pathways for evidence grading, indicator mapping, and uncertainty treatment. The proposed framework can be applied to protected areas, ecological conservation redlines, ecological restoration projects, biodiversity offsets, and regional ecological spatial governance.