Abstract: It was found that when the concentration of co-existing phophorus was larger than 0.1 mg·L-1,negative deviations of results of calcium in AAS determination were obtained as measured at 422.7 nm,and the magnitude of decrease in absorbance of calcium was increased with the increase of concentration of co-existed phosphorus.It was also found that when the concentration of phosphorus co-existed was in the range from 0.1-6.0 mg·L-1,an obvious correlationship was observed with the magnitude of negative deviation of results of calcium.Algorithms of back propagation artificial neural network (BP-ANN) and simulated annealing (SA) were applied to study the non-linear relationship of the above mentioned interference effect,and a method of simultaneous AAS determination of Ca and P by measuring at a single wavelength of 422.7 nm was proposed.The proposed method was applied to determination of Ca and P in cycling water samples in the ranges of 0.08 to 10.0 mg·L-1 (for Ca) and 0.10 to 6.0 mg·L-1 (for P) and values of recovery found were in the ranges of 100.5% and 98.0% respectively.