Abstract: During the period of spring bloom from March 3 to April 16, 2005 in Xiangxi Bay, The vertical attenuation coefficients(KSd) of downward visible irradiance(PAR) and Secchi disc transparency(Sd)were measured with a scanning spectroradiometerand a 30 cm diameter black and white quadrant disc. Water chemistry variables such as chlorophyl-l a and dissolved organic carbon(DOC) weremeasured to determine the relative contribution of attenuat ing constituent in explaining the substantial variation inthe PAR attenuation coefficients KSd. Based on the in situ investigat ion data obtained, The underwater light field in Xiangxi Baywas characterised and the relationship between these two optical properties of the underwater was quantified with SPSS software.The analyses including application of empirical theories demonstrate that temporal patterns and inter-system differences in KSd and1/Sd during the period were well correlated for most of the sampling sites, except in the Xiakou reach, and the inverse relationshipthese two optical properties is influenced by chlorophyl-l a and inorganic tripton to some extent. Regression Analyses of thesePAR attenuation coefficient data and observations of Chla and DOC, documente that Chlorophyll a and Dissolved organic carbonare the important constituents responsible for spatial distribution and temporal variation of the PAR attenuation coefficients KSd inXiangxi Bay, except in the Xiakou raech. In addition to application of empirical and deterministic modeling frameworks, thoseresults demonstrate that the Xiangxi Bay in the optic characteristic during the period of spring bloom is similar to many deeplakes, and differ from many shallow lakes where increase of total suspended matter due to sediment resuspension results in decreaseof transparency and increase of beam attenuation coefficient.