S-shaped bend waveguide is indispensable to realize the lateral displacement and connections in the integrated optical devices. The transmission losses of different S-shaped waveguides are analyzed, and the lowloss cosine shape mode is found. On the basis of the previous Marcuse theoretical conclusions of bending waveguide, a curve fitting method is used and several approximate loss calculation formulas are obtained. By using the Matlab software, the loss graph is gotten. Then compared with the traditional low- slope model, the closest one to the theoretical model is found. The results show that the Guassian curve fitting method can better describe the cosine-shaped waveguide loss.