Circular engineered cardiac tissue was fabricated by embedding rat embryonic cardiomyocytes into collagen (type I) gels. The engineered tissue was set to a specific configuration and the spontaneous beat displacement at one site of it was measured. The active contractile force of the embedded cardiomyocytes was derived from the displacement data. In this process, the engineered tissue was constitutively modeled as three components in parallel: i.e., an active contractile component representing the cardiomyocyte contraction, a pre-force component representing the effects of gel compaction during the tissue fabrication, and a Kelvin model for the passive properties of the tissue. Dynamic analysis of the beat displacement allowed solving out the active contractile force. In addition, energy coefficient was defined to evaluate the pump function of the engineered tissue. It demonstrated that this approach can detect the active contractile force as small as ∼0.01 mN and can sensitively reveal the change of the active contractile force under different culture conditions. Besides being an assay to evaluate the mechanical performance of engineered cardiac tissue, this novel method is particularly suitable to be used in pharmacological response testing of stem cell-derived cardiomyocytes under three-dimensional culture attributed to its high sensitivity and feasibility for continuous and in situ measurement.