In this paper, we propose a wavelet collocation method with nonlinear companding to generate behavioral models for analog circuits at the system level. During the overall process of circuit modeling, nonlinear function approximation is an important issue to accurately capture the nonideal input-output relations of analog circuit blocks. While a great number of previous research works focus on the high-dimensional top-down design/synthesis model, which involves large analog design spaces, this paper primarily concentrates on the bottom-up verification model requiring both simple representation and high accuracy. Taking advantage of the local support of wavelet bases, a nonlinear companding method is developed to control the modeling error distribution based on system-level simulation requirements. It, in turn, significantly improves the simulation efficiency at the system level. To demonstrate the promising features of the proposed method, two circuit examples, a fourth-order switched-current filter and a voltage-controlled oscillator, are employed to build the behavioral models.