The Lyapunov exponents of a DC-DC converter with constant power load cannot be calculated at the switching points because of the inexistence of Jacobi matrixes at these points. Aiming at this problem, this paper studies the Lyapunov exponent algorithm of such system using the local maps to construct the system compound Poincaré mapping. Thus, the piece-wise smooth nonlinear dynamic properties of the system can be analyzed according to the Lyapunov exponents. The calculating process is illustrated by the exa...