To detect the bulk defects in crystal billet of a KDP(potassium dihydrogen phosphate)crystal quickly and exactly, a laser focusing scanning scattering imaging method was proposed and a corresponding high speed line scanning measurement setup was constructed. The principle of detection, image acquisition, image processing and the extraction of bulk defects were investigated. Based on laser scattering technique, a focused line laser beam was used to scan all parts of the crystal billet with a high-speed movement device. The scattering light caused by bulk defects such as bubbles and inclusions was collected by a linear array CCD. The adverse effect caused by rough surface was eliminated with the index-matching fluid. Combined with digital image processing technique, the captured image was processed in real time. An averaged background was subtracted from the original image, then the image was compared with a threshold to judge the existence of bulk defects. The image with bulk defects was binarized to exact the positions and sizes of bulk defects. Finally, the setup was used to test the crystal billet of a KDP, results show that the sensitivity by proposed method is superior to 40 μm. It verifies that this method provides supports for accurately cutting and making the most use of crystal billet and also saves a lot of costs.