Under the effectives-mass envelope-function theory, the binding energy of the system in GaN/Alx Ga1-x N symmetric double quantum wells were theoretically calculated using the variational method. The influence of applied external electric fields, barrier height, quantum well width and the position of donors on the binding energies of donor impurities were investigated. The potential quantum well energy changes significantly with applied external electric field. The binding energy and wave functions with the donor in different positions were presented with and without external electric field. Variations of donor binding energy with the centre barrier width were also calculated. With the fixed middle barrier of double quantum wells, the binding energy increases until it reaches a maximum value, and then decreases as the well width increases. The results are meaningful in the design of optoelectronic devices based on GaN/Alx Ga1-x N quantum well structures.