This paper introduces an algorithm for detecting T wave alternans (TWA) using Poincare mapping method, which is a technique for nonlinear dynamic systems to display periodic behavior. Vector angle index (VAI) was used to determine the presence or absence of TWA. Adopting MIT/BIH Arrhythmia database and European ECG ST-T database to simulate, we used VAI via poincare mapping method for correlation analysis with Vtwa by way of the spectral method. The cross-correlation coefficient between Vtwa and VAI is gamma = 0.860 1. The algorithm can identify the absence and presence of TWA accurately and provide idea for further study of TWA.