The Silurian Longmaxi Formation to the east of the Kangdian Ancient Land in the Middle and Upper Yangtze Region has no red beds, but in the Yanyuan-Lijiang area to the west of the Kangdian Ancient Land, there are red beds (about 2 m thick purple red siliceous shale) at the bottom of the Longmaxi Formation. In order to make clear the genesis and geological significance of the red beds, this paper studies the red-black transitional characteristics from the red beds in the bottom of Longmaxi Formation to the black beds in the upper part of Longmaxi Formation, from the aspects of lithology, lithofacies and petrochemistry, taking the Dacaozi outcrop of Ninglang in Yanyuan-Lijiang area as an example. The results show that: (1) the "red bed" contains Fe3+ and there is no organic carbon in it, which may be the primary reason for its red color; (2) the characteristics of rare earth elements in "red layer" and "black layer" are similar, but the characteristics of trace elements are quite different, especially the mean value of Co content in "red layer" is more than 9 times higher than that in "black layer", and the mean value of Mo in the "black bed" is more than 9 times higher than that of the "red bed"; (3) from the "red layer" in the bottom of Longmaxi Formation to the "black layer" in the upper part of Longmaxi Formation, it is a process from relatively oxygen rich sedimentary environment to relatively anoxic sedimentary environment, and it is also a process that the sedimentary water gradually changes from shallow to deep, which is the main controlling factor of the red-black transition; (4) the tectonic setting from the lower "red bed" to the upper "black bed" is always in the active continental margin, which is different from the stable continental margin to the east of Kangdian Ancient Land.