A telescopic shoring system for shoring open excavations comprises rail posts composed by at least an outer member and an inner member, and large shoring panels. Each outer and inner member of the rail post has laterally, on either side, at least one panel guide allowing the shoring panels to slide within. The outer member of the rail post has a frontal guide adapted to interlock the back guide provided to the inner member but sliding relatively to each other. The shoring panels are arranged on each side of the excavation while the rail posts are placed on each corner holding on either side the shoring panels, which create a multi-stepped shoring box. The outer members of the rail post with shoring panels create an outer or upper shoring box while the inner members of the rail post in combination with shoring panels create an inner or lower shoring box.