The invention discloses a main structure for a metro long-span station and a pillar arching construction method thereof. The main structure comprises a station interior main structure, a plurality of middle pillars, a middle plate and a main arch part, wherein the station interior main structure is dug; the middle plate is located on the middle pillars; the main arch part is located right above the main structure; a bottom longitudinal beam is arranged at the bottom of the station interior main structure; a top longitudinal beam is arranged on the main arch part; and a middle longitudinal beam is arranged between adjacent middle pillars. The construction method comprises the following steps: 1) preparing in earlier stage; 2) digging a guiding hole and performing primary bracing and bridging construction; 3) constructing the bottom longitudinal beam, the middle pillars and the top longitudinal beam; 4) constructing a primary branch arch and refilling an arch back; 5) performing arch secondary lining construction; 6) digging the upper part of the main structure and constructing a primary branch of a side wall of the upper part; 7) performing secondary lining construction of the middle longitudinal beam, the middle plate and the side wall of the upper part; 8) digging the lower part of the main structure and constructing the primary branch of the side wall of the lower part; and 9) performing secondary lining construction for a base plate and the side wall of the lower part. The construction method provided by the invention has the advantages of reasonable design, simple construction step, convenience in realizing, small construction difficulty and high construction speed.