Abstract: Longitudinal ultrasonic intensive defects were tested for a batch of GCr15 steel large forging stocks and the equivalent defect diameter was higher than the requirement of GB/T 6402-2007 for the third quality grade. Because the black surface roughness of the GCr15 steel forging stocks was rather high, the ultrasonic testing results appeared forward system error and the actual testing results was higher than the real equivalent defect diameter. A method of μ-value correction was proposed, and for GCr15 steel forging stocks, the interference of black surface roughness on ultrasonic testing results could be eliminated by correcting with μ=1/10β/40. The results show the calibration to eliminate the interference of black surface roughness was rather good.