The process of the forward Langmuir waves scatted by ion sound waves is very important for the fundamental and harmonic emission of the type III solar radio bursts. We use the particle in cell simulation to study the property of the ion sound waves during the plasma emission process. We calculate the electric field only excited by ions during the simulation to study the evolution of the ion sound waves. We calculate the energy of ion sound waves with different parameters, and find that the temperature and mass of ion are important to enlarging the ion sound waves. Also, we verify the relationship between the ion sound waves and the backward propagating Langmuir waves due to the energy ratio of forward Langmuir waves and ion sound waves. Both the waves decay and scattered process have been confirmed. Therefore, we find the ion sound waves have a positive contribution to backward Langmuir waves.