Based on the image detection of steel ball surface defects，the accuracy and efficiency of detectionare seriously impacted by the effective area selected improperly. Firstly，the surface detection of steel ball optimalrange is studied to determine the ideal arc length;secondly，in view of the sphere zone theory establishes effectivearea and zonal overlapping relationship model，in order to restrict detection times by reducing the overlap area;finally，the formula of effective arc covering efficiency is utilized to directly control arc length redundancy caused bythe overlap area to improve the detection efficiency and laid the important theoretical basis for system developmentof ball diameter series detection.