A new method based on symbobic dynamics and relative entropy theory is proposed to examine the nonlinear behaviours of converters. Firstly, the discrete numerical sequence is obtained from iteration map, which is then transferred to a symbol-time series according to the topological conjugation, and the relative entropy is calculated by means of forward and backward probabilities. This paper takes a first one-order voltage feedback DCM Boost converter as an example, and the result shows that the relative entropy, which can measure quantitatively the distance apart from equilibrium when converter lies in a chaotic state, is a new and quantified nonlinear dynamic behaviours which has not been used in converters yet.