We propose a simple and intuitive method to derive the exact convergence rate of global $L_2-norm$ error for strong numerical approximation of stochastic differential equations the result of which has been reported by Hofmann and $M\text{"}uller-Gronbach\left(2004\right)$. We conclude that any strong numerical scheme of order $\gamma >1/2$ has the same optimal convergence rate for this error. The method clearly reveals the structure of global $L_2-norm$ error and is similarly applicable for evaluating the convergence rate of global uniform approximations