Steel and concrete composite girder began to be used in arch bridge. For the existing of shear lag, the concrete deck in same cross section did not bear identical stress completely. No stipulations for effective width of concrete deck of composite beam in arch bridge are given in current design specifications and codes, which causes difficulties in analyzing the mechanical behavior of composite arch bridge by beam-column method. In this paper, the mechanics calculation for a multi-span arch bridge with continuous composite beam is conducted. Stresses distribution pattern of concrete deck in transverse direction and the variation of effective width of concrete in longitudinal direction are studied. The degree of impact of effective width caused by varying loads on structure and by varying some details on structure is discussed.