The invention discloses a detection method for the strength of a building material. The detection method comprises the following steps: S1, acquiring comparison data: taking three groups of building stones which are just qualified after being subjected to strength detection, and naming each group of the building stones as x1, x2 and x3, wherein the mass of each group of the stones is the same; after cleaning up and drying the stones, weighing respectively; after weighing, obtaining the mass of the three groups of stones as m1, m2 and m3 respectively; then putting the three groups of stones into large measuring cylinders filled with clean water respectively, and immersing the stones with the clean water in the measuring cylinders; observing liquid level changes before the stones are put into the large measuring cylinders and after the stones are put into the large measuring cylinders. According to the detection method disclosed by the invention, comparison detection can be carried out on the strength of the building stones in a laboratory of a construction site through density comparison, so that the strength of the building stones can be rapidly detected; the stones do not need tobe conveyed into a large laboratory for detection; the detection of the strength of the stones is simplified; meanwhile, the construction period is saved.