In order to solve the problem of how to determine the coupling strength in coupling synchronization method,a theorem of single coupling method between Lorenz systems is proposed. First, by reducing the number of state variables, an improved synchronization method named，i.e. single coupling method, is proposed on the basis of the unidirectional coupling synchronization Then, in accordance with stability theory and Routh-Hurwitz criterion, by detecting the signs of characteristic values reduced from the Jacobi matrix of error system, full conditions are derived for satisfying the coupling coefficient in error gradually stabilizing error at zero. Consequently, a sufficient condition of single coupling synchronization is supposed. Last, the simulation results verify the correctness and effectiveness of the sufficient conditions, and the anti-interference performance is good.