The invention discloses a method for detecting spangle sizes on the surfaces of hot-dipped galvanized sheets on line for improving the detection efficiency and the precision. The method comprises the following steps of: passing continuously produced hot-dipped galvanized sheets through a roughness detector to obtain filtering waviness values; and obtaining the spangle sizes on the surfaces of the hot-dipped galvanized sheets according to the value range of the filtering waviness values. According to the method, the filtering waviness Wca values on the surfaces of the hot-dipped galvanized sheets are detected through the on-line roughness detector to obtain the spangle sizes on the surfaces of the hot-dipped galvanized sheets, and a technical condition is provided for the on-line quick judgment and adjustment of the spangle sizes on the surfaces of the hot-dipped galvanized sheets, particularly for the control of the process speed of the hot-dipped galvanized sheets, so that the uniformity of zinc layers can be reasonably controlled, and the measurement errors of the filtering waviness Wca values can also be furthest reduced so as to improve the detection precision.