In wavefront compensation systems, a compensation cycle includes three basic procedures, namely wavefront detection, generation of correction command and response of the corrector. On considering that the coherence time of the atmosphere is limited, and it should usually take some limited time to complete each of these procedures one after another successively when digital computation is involved, a step-by-step model should be suitable to study the performance of active systems. Based on that model, the statistical optical transfer function (SOTF) of the system is evaluated. SOTF is first associated with the structure function of the compensated wavefront. The latter is then associated with its detection error, the corrector setting error, the spatial response function of atmosphere, under some assumptions. The results are checked for severl special cases. It is found that they are either identical with those published by other authors or quite reasonable and easily explainable.