We present a method for the computation of Lyapunov exponents without reorthogon alization. In the low dimension of system(n<5) ,the equations needed in present algorithm is less than those in normal methods such as QR, SVD etc. This method is applicable to both discrete systems and continuous systems, and is still valid when the Lyapunov spectra is degenerate. Numerical analysis to Lorenz dynamical system indicates that the method converges quickly and steadly for arbitrary nonzero initial state.