The characteristics of the switching curves of Gaussian-Constant-Gaussian varying coupling coefficient nonlinear directional coupler (GCG NLDC) are analyzed in detail. The influences of the parameters of the coupler's subsections on the switching curves are studied. It is found by numerical simulation that the existence of sidelobes of the switching curves and the value of the switching powers are related to the parameters of all the subsections. The magnitude of sidelobes of the switching curves is decided by the relative length of bent coupler L/b and the quotient of conventional NLDC, the sidelobes of the switching curves have close relations with the oscillation of the Fourier transforms of the coupling functions.