A new type rhodanine ramification 3-(4'-methoxyphenyl)-5 (2'- arsenoxylphenylazo)- rhodanine (4MORAAP) was prepared as fluorescent reagent in this paper. A new pectrofluorimetry method was proposed to determine the trace bismuth based on the reaction between potassium periodate and 4MORAAP. The fluorescence intensity was found to be quenched by the oxidation of 4MORAAP by potassium periodate with bismuth as catalyst in the buffer medium of potassium hydrogen phthalate-sodium hydroxide (pH=5.2). Under the optimum conditions, the fluorescent intensity was correlated to be linear with the concentration of bismuth over the range of 2.0×10-7-1.8×10-6 g×L-1 and the detection limit was 2.9×10-8 g×L-1. The relative standard deviation (RSD) was 2.4 % for determined1.0×10-6 g×L-1 bismuth (n=11). The method was proved to be sensitive and selective and was successfully applied to determine trace bismuth in domestic water.