Aggregate size determines core sample diameter in the concrete strength inspection by core drilling method，and the core sample diameter is 2 times higher than the aggregate size.Taking the thin-walled member of irrigation and drainage engineering as an example，this paper has studied the test methods of concrete strength of the precast element which composed of fine aggregate with a diameter of 20mm.In addition，the relation between the core sample with a diameter of 40mm and the strength of 28d cube specimens with a dimension of 150 mm has been studied.The logarithmic function relation and corresponding curves of the core sample with a diameter of 40mm have been put forward.It provides reference for the test methods of concrete strength of small diameter precast element．