In order to obtain the optimum red edge parameters for using rapid detection of the lettuce leaf nitrogen content, the response feature of red edge parameters for leaf spectra under different nitrogen levels and the relationship between red edge parameters and leaf nitrogen contents were investigated. Visible and near infrared (Vis—NIR) spectroradiometer was used to collect the leaves’ spectral data of water culture lettuce, and the chlorophyll meter (SPAD 502) was used to measure the leaf chlorophyll content. Red edge parameters, which included red edge position (λred), red edge area (Sred), red edge differential (Dλred), the minimum of red edge differential (Dλmin) and the ratio of Dλred to Dλmin (Dλred/Dλmin), were calculated based on the first derivative reflectance spectra in the red edge region. The results showed that the dynamic characteristics of red edge parameters changed with different nitrogen nutrition levels in water culture. It was concluded that λred, Dλmin,Sred and Dλred/Dλmin had significant relationship with the leaf chlorophyll content, respectively. In all red edge parameters, the correlation degree of λred was the highest with correlation coefficient 0.9420. And the root mean squared error was the lowest with 1.803. Therefore, it was demonstrated thatλred could be a predictor for lettuce leaf nitrogen content.