A novel concept of switching block is presented via detecting the switching behavior of each switching period in piecewise smooth system of DC-DC switching converter, and logical arithmetic method of computer is used to obtain decimal system from binary system, so complex degree of the system during one switching period is quantized as switching block and the symbolic time sequence of the piece smooth system is established. Weight Lempel-Ziv (L-Z) complexity is derived based on the L-Z complexity and the symbolic time sequence, and the qualitative analysis of nonlinear and complex degree in piecewise smooth system from symbolic time sequence with single variable is carried out. The motion rule and the dynamics structure of the whole system are revealed. Finally, The present current mode controlled buck converter is studied as an example to establish symbolic time sequence and to illustrate the applications of weight L-Z complexity.