In the process of EL testing of p-type crystalline silicon solar cells, black spot occurs in part of cells. Combining with X-ray energy spectrum analysis (EDS), contrastive analysis on the black spot cells and normal cells indicates that most of the surface component of the black spot cells and the normal cells is the same, which excludes the possibilities of appearing black spots in the process of coating and screen printing. Tests performed on the shading area and the normal area from the same cell by X-ray fluorescence spectrometry (XRF) show that Ca content is larger in the shading area, even other impurity elements as Sr, Ge or S appearing in the same shading area. External quantum efficiency(EQE) of each cell was teated by measuring the light-induced current, and it showed that there is a big difference between the shading and the normal areas within the wavelength range of 460~1000nm, which indicates that not the primary silicon defects but the impurity defects introduced in the production process of solar cells leads to the black spot. The causes of impurities and solutions were given, significantly reducing the production probability of black spot cells.