Effectual stress system, as the bridge of relating stratum energy and CBM reservoir formation, is formed by macroscopical dynamical energies which act on coalbed. The main influencing factors of effectual stress system are tectonic stress, groundwater head height and groundwater mineralization. Researches of the effectual stress system in the southern Qinshui basin reveal that the key phase influencing on CBM reservoir forming and destroying was from the Late Jurassic to late Early Cretaceous, during this period the state of effectual stress system was continually transition between opensystem and closesystem, and formed a great deal of holes and crannies, which not only established the basis of effectual carrying system, but also foreshadowed the potential exploitation of high rank CBM reservoir in southern Qinshui basin.