The effect of the plasma density structure on the electron energy in laser wakefield acceleration is studied. Using a simple rotatable supersonic nozzle, the plasma density structure is controlled. Two dimension particle in cell simulation indicates that due to the decrease of the size of the acceleration field cavity with increasing density structure, the electron energy is enhanced. Using 20 TW laser system, the laser wakefield acceleration with density increment structure was conducted. The results show the enhancement of the electron energy with the density increment structure.