The root length density (RLD) is an important parameter to model water and nutrient movement in the vadose zone and to study soil–root–shoot–atmosphere interactions. However, it is difficult and time-consuming to measure and determine RLD distributions accurately. Especially RLD distributions change with different soil environment, plant species, growing seasons, and climatic conditions. In this study, measured data sets of wheat RLD distributions were collected from the literature and transformed into normalized root length density (NRLD) distributions. A total of 610 values of wheat NRLD distributions were pooled together. These data showed a general trend, independent of soil environment, wheat species, growing seasons, and climates. A generalized function was established to characterize the NRLD distributions versus normalized root depths. To verify the generalized function, we measured RLD distributions of winter wheat (Triticum aestivum L.) using laboratory and field experiments for different soils, growing stages of wheat, atmospheric conditions, and water supplies. Using the generalized function, we predicted winter wheat RLD and compared the predicted results with the experimental data and with results using other NRLD functions. The comparison showed that the generalized function predicted RLD distributions more accurately than the other functions. Although simulated results of soil water dynamics in soil–wheat systems were similar for the different NRLD functions, the generalized function should be advantageous for applications that require accurate information of root development and distribution.