Ammonia nitrogen is an important indicator to measure the quality of surface water. In the process of detecting ammonia nitrogen content with Nessler’s reagent spectrophotometry method (HJ535—2009), water sample pretreatment (flocculation method), residual turbidity and instrument baseline drifting will bring uncertainty to the process. Accordingly, a spectroscopic method based on dual-wavelength (420 and 650 nm) measurement is proposed, so as to subtract the absorbance of residual turbidity and instrument baseline-drifting to eliminate such interference. We first figure out k, the turbidity correction coefficient of the water sample at the wavelength 420 nm, then divide the product of absorbance of 650 and k by the absorbance of 420, and finally obtain the net absorption of ammonia nitrogen chromogenic reaction. Thus we can make accurate quantitative detection of ammonia nitrogen content and evaluate the precision and accuracy of dual-wavelength spectrometry. The results shows that, with single-wavelength method, the relative deviation can be up to 8.67% caused by the filtration process of flocculation and sedimentation, while the dual-wavelength method would not be affected by the deviation, since dual-wavelength method includes no filtration process. The accuracy standard deviation of dual-wavelength method could be as low as 1.58%, and the recovery is between 98.5% and 103%, which shows that the method is more accurate and reliable. Compared with the current Nessler’s reagent spectrophotometric method, the present method not only omits the process of water sample pretreatment, but also avoids the interference of residual turbidity, which could significantly improve the efficiency of the experiment. Therefore, it is more suitable for rapid determination of ammonia nitrogen in large quantities of surface water samples.