In order to improve the detection sensitivity and efficiency of soil available phosphorus，the detection methods for extraction of available phosphorus by sodium bicarbonate in HJ 704—2014 Determination of soil available Phosphorus by sodium bicarbonate leaching-molybdenum antimony resistance spectrophotometry and LY/T 1232—2015 Determination of Forest soil were optimized. By designing the reagent preparation scheme and reagent flow path of continuous flow analyzer，the extraction method，oscillation velocity，temperature，time and detection wavelength on the test results were studied，the linear range，detection limit，precision and accuracy of the method were investigated，and the detection results of spectrophotometer method and continuous flow analyzer method were compared. The results showed that at 880 nm wavelength，the linear range of effective phosphorus extracted by sodium bicarbonate leaching was 0-3.00 mg/L，the correlation coefficient R² ＞0.999，the detection limit was 0.004 mg/L，and the coefficient of variation was＜5.0%. The reference substance NSA 2[validated effective P（36±6）mg/kg]，NSA 4[validated effective P（14.5±2.6）mg/kg ] and standard substance ASA-3a[validated effective P（29±3）mg/kg]were measured as 35.9，12.5 and 28.1 mg/kg. The optimized continuous flow analyzer method for the detection of available phosphorus content in sodium bicarbonate leaching soil had the advantages of automatic sampling，online bubble removal，online constant temperature color detection，less reagent consumption and environmental friendliness. Besides，the experiment period was short，and the measured data were of good sensitivity，precision and accuracy，showing outstanding advantages. It could provide technical basis for the effective phosphorus content of sodium bicarbonate leaching in the third national soil survey.