Solidification behavior in the mold region plays an important role in production efficiency and steel quality. To investigate shell growth within a mold, the sulfur prints of the entire shell thickness profile from the meniscus to 100 mm below the mold were obtained by adding FeS tracer into molten steel during bloom continuous casting of hypo-peritectic steel. The law of shell thickness evolution along mold height and circumference was analyzed. The results show that there are three weak regions of solidification, which are in the mold upper part, in the mold lower part, and just below mold exit, possibly resulting from periodic fluctuation of air gap between the shell and the mold, the impingement of melt jets on the solidifi-cation front, and the decreasing cooling intensity, respectively. Initial solidification point along casting direction appears at approximately 35 mm below the meniscus. Overall, the solidified shell thickness in the inner side of the mold is a little larger than that in the outer side, and the former and the latter reach 25.5 and 24.3 mm at the mold exit, respectively. The non-uniform shell growth in the inner side of the bloom is provided, while shell thicknesses in the narrow face and the outer side follow relatively regular growth. Out of the mold, the thinnest shells on the transverse section exist in the regions of 60–90 mm and 40–70 mm from the cers of the inner and outer sides, respectively, i.e., the off-cers.