The invention provides an in-line cutting device used for seedlings. The device comprises a support, a seedling pit plate conveying mechanism, a holding clamp, a left supporting clamp, a right supporting clamp, a cutting rack, a seedling guide plate, a seedling collecting rod and a supporting clamp fixing plate; the support is installed above the seedling pit plate conveying mechanism in a bestriding way; the seedling guide plate is installed on the support, and the seedling collecting rod is arranged above the seedling guide plate; the supporting clamp fixing plate is installed on the seedling guide plate in a sliding way; the left supporting clamp is installed on the supporting clamp fixing plate in a sliding way; the right supporting clamp is installed on the supporting clamp fixing plate in a sliding way; the seedling cutting rack is installed on the supporting clamp fixing plate in a sliding way; and the left supporting clamp and the right supporting clamp are matched with the holding clamp to perform seedling in-line cutting operation. The device can finish the seedling in-line clamping, the holding cutting and the adjustment of the angle of the cutting rack.