The present invention discloses a calculation method of movement authority for communications-based train control system, comprising: handling a route information for a train, and determining a searching range of the train according to the route information; initializing the limit of movement authority with the end position of the searching range; searching for static obstacles within the searching range, and successively determining whether each static obstacle meets the safety requirements for train operating, if not, setting the position of the last static obstacle within the searching range as the limit of the movement authority; if so, modifying the limit of movement authority as the end of route having been matched; searching for dynamic obstacles within the searching range, and determining whether there is a train, if so, modifying the end of movement authority as the beginning point of the track section where the train is occupying; if there is no dynamic obstacle within the searching range, modifying the final end is of movement authority as the position of the last static obstacle within the searching range. In accordance with the present invention, it is possible to increase line capacity and improve traffic fluidity for rail transit.