The invention discloses a longitudinal seedling conveying mechanism of a transplanter, which comprises a seedling box. The two sides of the seedling box are provided with support plates of seedling pressing rods. The inside of the seedling box is provided with a vertically conveying mechanism which is connected with a cam drive mechanism and consists of a primary vertically conveying mechanism and a secondary vertically conveying mechanism. The primary vertically conveying mechanism and the secondary vertically conveying mechanism are provided with a primary roller wheel and a secondary roller wheel respectively. The cam drive mechanism acts on the primary vertically conveying mechanism, and the primary vertically conveying mechanism and the secondary vertically conveying mechanism are linked by a transmission component arranged between. Pot mat seedlings, common mat seedlings and other various seedlings are conveyed through positioning of the roller wheels of the primary vertically conveying mechanism and the secondary vertically conveying mechanism without being piled. Seedling paws can catch the seedlings accurately and keep the overall shape of the seedlings from being damaged and the seedlings conveyed out in order.