Abstract: Focuses on the problem of ranging error diverges with the increase of the starting incidence angle and non-convergence of iteration process at the large incidence angle by using the traditional method to calculate the starting incidence angle, this paper proposes a new method of calculating the starting incidenceangle by Newton method. Firstly, the principle and steps of traditional method are introduced. Secondly, the iteration equation is established according to the relationship between the propagation time and the starting incidence angle. Finally, the two methods are compared by two simulation experiments in deepsea and shallow sea. The results show that the computational efficiency of the traditional method is higher than our proposed method when the starting incidence angle is less than 78°. In addition, it is found out that when the starting incidence angle is greater than 60°, the ranging error calculated by the traditional method begins to diverge. In the shallow water simulation experiment, the traditional method also has the problem of non-convergence in the iteration process when the starting incidence angle is greater than 85°. In contrast, our proposed method can converge to the real value at any starting incidence angle.