A method for manufacturing a wind turbine blade, comprising the steps of: pre-manufacturing a first blade member, including a spar cap (13, 14), said spar cap (13, 14) comprising composite material with fibers running in a longitudinal direction of the blade, positioning said pre-manufactured first blade member in the center of a joining mold (12, 12a, 12b), and bonding said first blade member with a second blade member (21a, 21b, 23, 24) using a vacuum assisted infusion process so as to form an integrated blade part, wherein the second blade member is placed next to the pre-manufactured first blade member on each of its longitudinal sides respectively, wherein the first blade member comprises a pair of bonding flanges (16, 17) for bonding said first blade member to the second blade member, the pair of bonding flanges (16, 17) being disposed at a trailing edge side and a leading edge side of the first blade member, respectively, wherein each of the bonding flanges (16, 17) extends continuously along the longitudinal direction of the pre-manufactured first blade member and has a flat extension (16b, 17b), wherein the step of bonding the first blade member with the second blade member (21a, 21b, 23, 24) includes: arranging the pre-manufactured first blade member such that the flat extension (16b,17b) of the bonding flange (16, 17) adjoins the joining mold; and positioning blade building material (21a, 21b) forming the second blade member along the longitudinal sides of the pre-manufactured first blade member in such a way that the blade building material (21a, 21b) overlaps with the flat extension (16b, 17b) of the bonding flanges (16, 17) and then bonding the first blade member with the blade building material (21a, 21b) using a vacuum assisted infusion process, or positioning pre-manufactured second blade members (23, 24) such that a flange (23a, 24a) of each of the second blade members (23, 24) mates with the flat extension of the bonding flange (16, 17) of the first premanufactured blade member and then bonding the first blade member with the second blade members (23, 24) using a vacuum assisted infusion process. pre-manufacturing a first blade member, including a spar cap (13, 14), said spar cap (13, 14) comprising composite material with fibers running in a longitudinal direction of the blade, positioning said pre-manufactured first blade member in the center of a joining mold (12, 12a, 12b), and bonding said first blade member with a second blade member (21a, 21b, 23, 24) using a vacuum assisted infusion process so as to form an integrated blade part, wherein the second blade member is placed next to the pre-manufactured first blade member on each of its longitudinal sides respectively, wherein the first blade member comprises a pair of bonding flanges (16, 17) for bonding said first blade member to the second blade member, the pair of bonding flanges (16, 17) being disposed at a trailing edge side and a leading edge side of the first blade member, respectively, wherein each of the bonding flanges (16, 17) extends continuously along the longitudinal direction of the pre-manufactured first blade member and has a flat extension (16b, 17b), wherein the step of bonding the first blade member with the second blade member (21a, 21b, 23, 24) includes: arranging the pre-manufactured first blade member such that the flat extension (16b,17b) of the bonding flange (16, 17) adjoins the joining mold; and positioning blade building material (21a, 21b) forming the second blade member along the longitudinal sides of the pre-manufactured first blade member in such a way that the blade building material (21a, 21b) overlaps with the flat extension (16b, 17b) of the bonding flanges (16, 17) and then bonding the first blade member with the blade building material (21a, 21b) using a vacuum assisted infusion process, or positioning pre-manufactured second blade members (23, 24) such that a flange (23a, 24a) of each of the second blade members (23, 24) mates with the flat extension of the bonding flange (16, 17) of the first premanufactured blade member and then bonding the first blade member with the second blade members (23, 24) using a vacuum assisted infusion process. arranging the pre-manufactured first blade member such that the flat extension (16b,17b) of the bonding flange (16, 17) adjoins the joining mold; and positioning blade building material (21a, 21b) forming the second blade member along the longitudinal sides of the pre-manufactured first blade member in such a way that the blade building material (21a, 21b) overlaps with the flat extension (16b, 17b) of the bonding flanges (16, 17) and then bonding the first blade member with the blade building material (21a, 21b) using a vacuum assisted infusion process, or positioning pre-manufactured second blade members (23, 24) such that a flange (23a, 24a) of each of the second blade members (23, 24) mates with the flat extension of the bonding flange (16, 17) of the first premanufactured blade member and then bonding the first blade member with the second blade members (23, 24) using a vacuum assisted infusion process.